Free-hand paint a big beach scene, or try something as simple as adding a stenciled sailboat, anchor, or helm wheel above each towel hook. (Personalize it for the kids in the house with their first names under each stenciled object.) Or stencil small starfish motif around a mirror. You can find many nautical themed stencils at craft stores.
Add a great clean coastal look-tongue and groove walls. Go part way up the wall and use a chair rail, or go all the way up!