> I still have the question about the origin of the Greek word for sea,
> though: was thalassa/thalatta a native Greek word?
We were taught that it was a loan word, from some unidentified Pelasgic
language, along with all those -lith words. As far as I know, this is still
the consensus. The ending however is what we would expect from a noun
ending in a voiceless dorsal or apical, with a suffix Proto-Greek *-ya <
PIE *-iH2. So if it's a loan word, it has been pressed into a recognisable
Greek declension pattern, which is hardly a surprise.
Peter