Sea - Trench's New Testament Synonyms
Seathalassa (G2281) Sea
Curtius and Pott connect the noun thalassa with the verb tarassein (G5015) and consequently define the former as "agitated" or "disturbed." Schmidt dissented and urged that the sea's predominant impression on the beholder is not one of unrest and agitation but of rest and quietude. According to Schmidt, thalassa refers to "the sea as a huge body of saltwater in view of its natural consistency and in view of its meaning; it is not any different from the brine [hals] mentioned in poetic literature." Schmidt also called the sea "the great salt flood." Without further discussion of this point, suffice it to say that like the Latin mare (sea), thalassa is the sea as contrasted with the land or, more strictly, with the shore.
Pelagos is the vast uninterrupted expanse of open waterthe altum mare (high sea)as distinguished from a sea broken by islands and shut in by coasts and headlands. Pelagos primarily suggests the breadth of the open sea and not its depth, except as a secondary notion. Thus Sophocles said: "The open sea [pelagos] here is large and not navigable." The murmuring Israelites likened pelagos to the illimitable sand-flats of the desert. In Herodotus (2.92) the Nile that overflows Egypt is said "to keep flooding [pelagizein] the plains," though it covered them in water only a few feet deep (cf. 2.97). Plato recognized this distinction between breadth and depth and refused to call the Mediterranean Sea pelagos, because he saw it as a harbor with the narrow entrance between the Pillars of Hercules as its mouth. Only the great Atlantic Ocean can be acknowledged as "a genuine sea, truly an open sea [pelagos]."
In Matthew 18:6, however, one of only two New Testament uses of pelagos, that distinction may seem invalid. Matthew 18:6 says: "It would be better for him if a millstone were hung around his neck, and he were drowned in the depth of thesea." But the sense of depth that the passage requires is found in katapontisthe (he were drowned), not in pelagos. Katapontisthe implies the sea in its perpendiculardepth, just as pelagos is the sea in its horizontaldimensions and extent. benthos (L-S 313, depth).