Use maritime to describe anything involving the sea and ships. A maritime museum would probably be located in a coastal town, and stuffed full of historic boats, oars, life preservers, and fishing gear.
Maritime comes from the Latin word maritimus, which means "of the sea," so the meaning hasn't evolved much. Whales and dolphins are maritime animals, and according to maritime law, the captain of a ship can marry two people at sea. It's very similar to nautical, except nautical refers to ships, and maritime covers ships and other ocean-related stuff.