Here are some relevant references from the Books where Thassa is mentioned.
I make no pronouncements on these matters, but report them as I find them.
Arrive at your own conclusions.
I wish you well,
I have included this section because this topic is yet another pet-peeve of mine. Specifically the fact that Thassa is an entity of itself. It is not a thing.
Too many people refer to Thassa in a sentence like this: "The Vosk delta empties into the thassa." Or "The thassa is the sea on Gor." Or even something as simple as "Thassa means the sea".
Thassa is, instead, a personal pronoun. That means it is a name.
Hopefully you would not refer to me as "the Fogaban". That is not proper grammar.
And while I don't really care what you call me, it shows great disrespect to Gor and Gorean knowledge to refer to Thassa in any other manner than to consider it a living thing.
The word Thassa is never, ever shown in the Books as "the Thassa". What I mean is, the word 'the' never appears directly in front of the word 'Thassa'.
And the word 'Thassa' is always capitalized.
Read now through these references and notice the manner in which Thassa is mentioned.
Ko-ro-ba lay in the midst of green and rolling hills, some hundreds of feet above the level of the distant Tamber Gulf and that mysterious body of water beyond it, spoken of in Gorean simply as Thassa, the Sea.
Outlaw of Gor Book 2 Page 39
I wondered at the things she said to me for they seemed strange, perhaps more so to my ears than they would have to one bred and raised from infancy as a Gorean, one as much accustomed to the submission of women as to the tides of the gleaming Thassa or the phases of the three moons.
Priest-Kings of Gor Book 3 Page 204
I saw even a black larl, a huge catlike predator more commonly found in mountainous regions; it was stalking away, retreating unhurried like a king; before what, I asked myself, would even the black larl flee; and I asked myself how far it had been driven; perhaps even from the mountains of Ta-Thassa, that loomed in this hemisphere, Gor's southern, at the shore of Thassa, the sea, said to be in the myths without a farther shore.
Nomads of Gor Book 4 Page 2
The ashes, Kuurus judged, since the body had been wrapped in the scarlet leather of a tarnsman, would be scattered from tarnback, perhaps over distant Thassa, the sea.
Assassin of Gor Book 5 Page 3
It is said that men once having seen Thassa are never willing to leave it again, that those who have left the sea are never again truly happy.
Assassin of Gor Book 5 Page 240
I could smell the sea, gleaming Thassa, in the myths said to be without a farther shore.
I reached down from the rush craft and took a palm of water into my hand and touched my tongue to it. Thassa could not be far beyond.
Raiders of Gor Book 6 Page 1
I expected I would again, however, return to Thassa. She, as it is said, cannot be forgotten.
Raiders of Gor Book 6 Page 140
In Se'Kara, particularly later in the month, there are often high seas on Thassa.
Raiders of Gor Book 6 Page 250
He was bound, traveling over the hills and meadowlands east and north of Ko-ro-ba, for the city of Laura, which lies on the banks of the Laurius river, some two hundred pasangs inland from the coast of the sea, called Thassa.
Captive of Gor Book 7 Page 59
One of the girls in the frame lifted her head, and, miserable, surveyed our ship, off shore, on the green waters of Thassa.
Hunters of Gor Book 8 Page 19
There was a salt smell in the air, swift and bright in the wind. Thassa was beautiful.
Hunters of Gor Book 8 Page 247
I looked down into Thassa. The glittering surface of the water, broken by the stroke of the oars, seemed to swirl.
Hunters of Gor Book 8 Page 292
"We of Port Kar," I said, "have little quarrel generally with those of Scagnar, but it is true that the ships of this Thorgard have preyed with devastation upon our shipping. Many men of Port Kar has he given to the bosom of Thassa."
Marauders of Gor Book 9 Page 95
"I have much fear," said Samos. I regarded him. I had seldom seen him so. I looked at the heavy, squarish face, burned by the wind and salt of Thassa, the clear eyes, the white, shortly cropped hair, the small golden rings in his ears.
Tribesmen of Gor Book 10 Page 16
The Vosk is a mighty river which flows westward, emptying into a vast rence delta, finding its way eventually to Thassa, the sea.
Slave Girl of Gor Book 11 Page 248
The world's end was said to lie beyond Cos and Tyros, at the end of Thassa, at the world's edge.
Beasts of Gor Book 12 Page 28
"The rain forests closed the Cartius proper for most civilized persons from the south," I said, "and what trading took place tended to be confined to the Ubarates of the southern shore of Lake Ushindi. It was convenient then, for trading purposes, to make use of either the Kamba or the Nyoka to reach Thassa."
Explorers of Gor Book 13 Page 17
This same standardization, in the region of the Tamber Gulf and south, along the shore of Thassa, tends to be effected by the golden tarn of Port Kar.
Savages of Gor Book 17 Page 120
The Dorna was a ship, a tarn ship, a ram ship, shallow-drafted, straight-keeled, single-banked, lateen-rigged, carrel-built, painted green, difficult to detect in the rolling waters of Thassa, out of Port Kar.
Blood Brothers of Gor Book 18 Page 304
"It is Thassa, the Sea, Mistress," said the girl.
"What sea is it?" I asked.
"That is how we think of her," said the girl, "as the sea, Thassa."
Kajira of Gor Book 19 Page 40
In all this time we had been gradually moving north and westward, slowly toward the coast, toward Thassa, the Sea.
Players of Gor Book 20 Page 230
The navies of Tyros and Cos, for most practical purposes, command the green waves of gleaming Thassa.
Mercenaries of Gor Book 21 Page 141
It must be remembered, too, that these were river galleys and, on the whole, smaller than the galleys of Thassa.
Renegades of Gor Book 23 Page 354
In time, perhaps a few months, it might even find its way to the Tamber, and, perhaps, in time, to the surgent green washes, the vast rolling swells, of Thassa herself, the sea.
Vagabonds of Gor Book 24 Page 246
Too, far off now, like the sounds of Thassa breaking on a distant shore, we could hear the crowds.
Magicians of Gor Book 25 Page 97
The first tarnsman turned the train westward. In that direction would lie Thassa, the sea, and perhaps the port of Brundisium.
Prize of Gor Book 27 Page 357
of the lovely island of Cos, in Thassa.
Kur of Gor Book 28 Page 141
It rained heavily that night, the storm coming in from Thassa.
Swordsmen of Gor Book 29 Page 73
"Ours," I said, "was the last patrol of the season. Thassa grows cold, and angry. I advise you to turn about and lay to port, if you have a port. This is no time to tempt the indulgence of Priest-Kings, no time to tempt the season, or the patience of Thassa.
Mariners of Gor Book 30 Page 97
Thassa was restless.
. . .
This was no time to be abroad on Thassa. Did they know so little of her moods, of her temper?
Mariners of Gor Book 30 Page 100
Thassa, subtle and minacious, welcoming and threatening, benignant and perilous, restless, sparkling, and dangerous, green, vast, intriguing, beckoning Thassa. It is easy to see how she calls to men, she is so alluring and beautiful, and it is easy, as well, to see how, with her might and whims, her moods and power, she may inspire fear in the stoutest of hearts. Be warned, for the wine of Thassa is a heady wine. She may send you gentle winds and shelter you in her great arms, bearing you up, or should she please, break you and draw you down, destroying you, to mysterious, unsounded deeps. In her cups you may find many things, the unalienable riches of moonlight on water, her whispering in long nights, against the hull, her unforgettable glory in the morning, the brightness of her noontide, the transformations of her sunset and dusk, her access to far shores, the sublime darkness of her anger, the lashing and howling of her winds, the force and authority of her waves, like pitching mountains. She is the love of the Caste of Mariners. She is a heady wine. Her name is Thassa.
Mariners of Gor Book 30 Page 191
I had heard of Port Kar. It was well to the north and west, where the waterways of the Vosk's delta drained into the Tamber Gull the city's sea walls fronting the gulf on the south, Thassa the sea, on the west. Was it not from the sea gates of Port Kar that the galleys of the dreaded Bosk, Bosk of Port Kar, clove the dark waters of restless Thassa?
Conspirators of Gor Book 31 Page 119
The moods of Thassa are capricious, and the might of her winds and waves prodigious.
Smugglers of Gor Book 32 Page 81
I looked back, briefly, at the cold beach, and the restless, shimmering expanse of Thassa, and the horizon beyond it.
Smugglers of Gor Book 32 Page 103
I was told, one does not take to Thassa, the sea, in the winter. Even in the summer, with her storms and moods, she is daunting, unruly, and dangerous. In the winter, I was told, it would be madness to venture amongst the swirling mountains of her waves, the cold and bitter hammers of her winds.
Smugglers of Gor Book 32 Page 252
I would learn the gravity of Gor, for it was on this world I now was, was less than that of Earth, the planet being somewhat smaller, though it would have more land surface than Earth, as it possessed only one mighty ocean, not two, that ocean being restless, turbulent, gleaming Thassa, the sea.
Plunder of Gor Book 34 Page 76
They are found in fresh water, but return, through the delta of the Vosk, to the salt water of vast, turbulent Thassa to spawn.
. . .
Gorean sharks, in their several varieties, of course, are much more common in the waters of Thassa herself, particularly near the shallower banks, where sunlight encourages the growth of plants, and the plants attract several varieties of smaller fish, the parsit and others, on which the sharks feed.
Plunder of Gor Book 34 Page 139
There is only one city I am aware of in which the caste of thieves is explicitly recognized, which is a port on the Tamber Gulf, bordering Thassa, the sea.
Plunder of Gor Book 34 Page 199
I do not know where, for the body hood, in the vicinity of Brundisium, that the tarn rider brought his immense, broad-winged beast to earth, but I think it must have been near the coast, probably south of the port, as we had come from Ar, for I heard gulls and smelled Thassa, the sea.
Plunder of Gor Book 34 Page 223
As my master continued his watch, or prolonged his inquiries, I was occasionally allowed out of the court, usually to fetch water from the stone steps under the Cloth Market Bridge, which, with other bridges, spanned the Lena, one of the two streams that flowed through the city and debouched into Thassa.
Plunder of Gor Book 34 Page 240
The lesser wharves, or piers, are the southern wharves or piers, where they are divided by the Dacia, as it feeds into Thassa.
Plunder of Gor Book 34 Page 243
"But what you may not know," he said, "is that Gor has more land surface than Earth. She has her turbulent, mighty Thassa, but she has no second, vast sea, like that you speak of as the Pacific."
"I know of Thassa," I said, "and of lesser lakes or seas, but none that are comparable to Thassa. "
Plunder of Gor Book 34 Page 326
Had these realities not been contrived in the innocent corridors of evolution, with no more thought or intention than the tides of Thassa or the orbits of worlds?
Plunder of Gor Book 34 Page 587