Unknown.
As you say, Tethys was never mentioned again in any later Discworld book nor mentioned by Pratchett in interviews. The Terry Pratchett Wikia has this to offer (emphasis mine):
Tethys is an alien to the Discworld, and the only sea troll on the Disc. He used to be a "meatman" (the water-world equivalent of a fisherman-- he hunted wild game on prairies) on his home world of Bathys, where he fell off the edge, froze solid, fell through space for some time and finally thawed out as he approached the Disc's Sun, and landed widdershins of Krull, where he was eventually captured and made a slave. He has considered jumping over the Edge to find his home world again, but has never found the courage.
Tethys' fate is unknown. It is presumed that he entered in the Krullian spaceship after Twoflower, but when Rincewind and Twoflower were brought back to the Disc by the Change spell, the sea troll wasn't there. Maybe he somehow found his way back to the world of Bathys and could live in peace ever after with his wives and children.
It's also possible that Pratchett later decided he didn't really want to have such a character and that this explains the lack of further mention of him (a 'passive retcon', if you will):
One can't help wondering whether TP would ever have written Tethys into existence by about book 5 - an elemental troll is not really in keeping with Discworld physics and chemistry, and he has never been mentioned since ...
