Watermaker: While these things aren't cheap, they will save you from dehydration by filtering out almost all of the salt in your drinking water.
Inflatable Life Vest: Just like the kind on airplanes. They are small and compact and inflate quickly.
One-Person Life Raft: These will fit into a suitcase and will keep you safe and sort of dry until the rescue choppers come.
Waterproof Passport Holder: You are banking on the fact that you will be rescued, right? Make sure to keep your passport safe to make your travel life easier after you are picked up.