See Definition for thalassa { [G2281]}
See Definition for pelagos { [G3989]}
thalassa is the more general word, indicating the sea or
ocean as contrasted with the land or shore. It may be applied to
small bodies of water.
pelagos is the open sea, the uninterrupted expanse of water, in
contrast with the portions broken by islands or with partly inclosed
bays. The prominent thought is said by Trench to be breadth rather
than depth. Noteworthy is the distinction between the two words in
Mt 18:6.