If you're asea, you're out on the ocean. When your tiny rowboat is asea, you'll have to paddle hard against the tide to make it back to shore.
Use the adverb asea to describe the direction "toward the sea," or to talk about something that's floating or sailing on the sea. When your friend explains why he moved to an island, he could say that he loves going asea on a ferry every time he needs groceries. The word is from the 1850s, a combination of a-, "to" or "toward," and sea, from its Old English root, sæ, "sheet of water."