Personally, I believe the Beast of the Sea to be Atlantis (or the government that was in place before the flood).

Beasts in prophecies are usually governments or civilizations. Anti-christ = horn on the beast.

So it makes since that the beast that rises from the Sea to be the civilization that was lost to the sea. Again, I think the ruler of that civilization (Azazel/baphomet) will be the anti-christ once he is released from the Abyss. However, keep in mind spirit beings can take on the form of flesh beings.