Those spongy, plastic key fobs will float, but not if you put more than three keys on them - I tested my old one in a bucket. As a serious boater, my key ring has only crucial keys on it, and I have eight: ignition, diesel cap, trailer lock, various locks on board and more. I took a good-sized empty vitamin-pill bottle with a screw top and spray-painted the bottle a bright color (paint goes on the inside if the container is clear; on the outside if it is white). Then I punched a small hole in the lid, pushed a piece of coat-hanger wire bent into a loop through the hole and sealed it with silicone. I attached my key ring to the wire loop and, for added security, I put a note inside with my name, address and phone number.