Rev 17:15 And he saith unto me, The waters which thou sawest, where the whore sitteth, are peoples, and multitudes, and nations, and tongues.
Answer: The “sea” here doubtless represents/symbolizes peoples, nations, tongues (languages), multitudes. Remember that John is on the isle of Patmos (Revelation 1:9) which is surrounded by water or the sea.
Example, “And I stood upon the sand of the sea, and saw a beast rise up out of the sea, having seven heads and ten horns, and upon his horns ten crowns, and upon his heads the name of blasphemy.” Revelation 13:1
In other words, John sees a beast rise up out of an area populate with “nations, people, tongues & multitudes”.