When something is described as seagoing, it's meant to travel in the ocean. A seagoing vessel might be an ocean liner, a submarine, or even a sturdy rowboat.
Your inflatable pool toy is probably not seagoing, but the ferry that takes you to a remote Maine island is. Someone who works on a ship or embarks on an ocean voyage can also be described with this adjective: "My ancestors were all seagoing people, so it makes no sense that I always get seasick on boats."