Revelation 12 speaks of a seven-headed, ten-horned Satan, which is not an actual monster but a figurative description of evil angels who oppose God. The seven-headed, ten-horned beast is an extension of the prophecy in Daniel 7, where Satan is described as coming out of the sea. 

[ Daniel 7:3] And four great beasts came up from the sea, diverse one from another.

The four beasts out of the sea (lion, bear, leopard, and ten-horned beast) foreshadow the ten nations that have divided from Babylon, Persia, Greece, and Rome. By describing Satan as a beast like this, God is showing that the angels ruling these nations are Satan himself. The angel who ruled Babylon was an angel named Lucifer, and the Persian and Greek empires were also ruled by evil angels, so the nations that began with Babylon and were renewed afterward were ruled by evil angels.

[Daniel 10:20~21] Then the angel said: Now do you understand why I have come? Soon I must leave to fight against the guardian angel of Persia. Then after I have defeated him, the guardian angel of Greece will attack me.  I will tell you what is written in The Book of Truth. But first, you must realize that no one except Michael, the guardian angel of Israel, is on my side.

Satan, as witnessed in Daniel 7 and 8, is depicted as a little horn, with the first little horn manifesting itself through the beast from the sea and the second little horn manifesting itself through the beast from the earth. The first and second beasts prophesied in Revelation 13 appear one after the other in the sea and on land, indicating an extension of Daniel's prophecy.

[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.

[Revelation 13:11]  And I beheld another beast coming up out of the earth; and he had two horns like a lamb, and he spake as a dragon.

God foreshadowed the rise of Satan from the beginning of time, and the great beasts of the sea and earth created on the fifth day recorded in Genesis 1 foreshadowed the rise of Satan.

[Genesis 1:20~24] God said, "I command the ocean to be full of living creatures, and I command birds to fly above the earth."...(24) God said, "I command the earth to give life to all kinds of tame animals, wild animals, and reptiles." And that's what happened.

So what do sea and land mean? In the history of creation, before the light appeared, the world was darkness, and before the land appeared, the world was the sea. However, this is not a record of an actual appearance, but rather a depiction of Jesus coming as light and the saints saved through Jesus as a land appearing out of light and sea. So to understand this, you need to relearn the six-day creation. The six-day creation, Daniel, and Revelation are not different prophecies, they are the same prophecy.