Most prophecy books that I've seen, use a symbolic approach to explain, "the sea", in Revelation 13:1:

Revelation 13:1 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.

Where the sea, often with restless waves, shows the condition of the restless nations of the world, or something similar to that.

So, if "the sea" in the second phrase is to be taken symbolically, then how is "the sea" in the first phrase to be taken?

How was John able to symbolically stand upon the very edge of the restless nations of the world?

Isn't it more likely that John stood upon actual, literal sand, on the edge of an actual, literal sea, like the Aegean Sea where Patmos was, or the Mediterranean Sea, Israel's west border?

If so, then would there be a literal sea in the first phrase, but a symbolic sea in the second phrase?

If they both are literal, then perhaps it may help identify where the antichrist arises from, e.g. like from the Mediterranean Sea?