Gas2.0 (an alternative energy blog) pointed out this video and gave some more background on a couple of different compressed air powered vehicles. The "non-piston" powered engine (starts about 5:30 into the video) sounds like it would have definite marine applications. They even mention the idea of using it on boats.
I don’t know about the efficiency of compressed air, but the safety aspects and ease of handling have some definite advantages. I hadn’t really thought of how much we use compressed air as a motive force already in industrial settings (tools and various other pneumatics). Maybe they have something here.