If you’re in a boat out on the water and you can’t see any land, then the body of water you’re in is probably vast, or enormous.
Things that are described as vast include oceans, seas, the heavens, deserts, and the surface of the moon. Even places that can't be seen or touched, like imaginations, are called vast, because of the endless ideas that come out of them. Someone with a vast imagination is very creative, and a place with a vast landscape is very large. Possibilities are vast, and so are the vocabularies of the world, filling vast volumes.