the sea: any large body of water, from the open ocean to an inland lake
Greek used one word for water on a large scale, and the New Testament does the same. Thalassa covers the wide saltwater sea that ships cross and storms churn, and it equally covers the freshwater lake of Galilee where Jesus did much of his Gospel ministry. The same word does both jobs, which is why English Bibles have to choose, verse by verse, between "sea" and "lake."
Most of the time the word is plainly physical: the water you sail on, drown in, or stand beside. When the New Testament reaches for an image of something vast or unmanageable (the countless sand of the seashore, a wave tossed by the wind, a mountain hurled into the sea), it is still the ordinary sea that supplies the picture.
The book of Revelation also leans on an older biblical association in which the sea stands for the restless, hostile deep, the place chaos comes from and where it is finally undone. That symbolic use is the same word stretched, not a separate word; it carries its force precisely because the literal sea is so familiar and so threatening.