Thalatta is the Attic dialect form of the word thalassa, meaning "sea."
It's a non-Indo-European loanword in Greek. That could mean that the ancestors of the Greeks had been living where there were no huge bodies of water, so when they invaded Greece they took over a word for "sea" from the previous inhabitants, whoever they were.
There are a number of loanwords in Greek with the -ss-/-tt- sound (glossa/glotta, "tongue," is another) along with many place names like Parnassos and Hymettos. They are also found on Crete (e.g., Knossos, Tylissos), which may mean that the pre-Greeks of the mainland spoke the same language as the "Minoans" on Crete.
So Attic hath a lithp like Castilian Spanish?