I live on a bayshore, and find that just keeping my bikes inside--like in a garage or storage shed--is the best protection. You want to keep them away from the fine, unnoticeable spray that inevitably makes itself into the air when you live near the water. As Knot has suggested, I've also used FrameSaver inside the frames, both of which are steel, and an occasional coat of wax on the outside. I've had to deal with some rust on hardware used to attach a rear rack, but otherwise they're both in pretty good shape, and one is about 7 years old now.

The Rubbermaid shed sounds like a good idea.