Waterproof Phone Case: Because it would suck to miss out on some ocean Pokemon Go action.
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.
I have no idea if that guy on the right is wearing pants and that is freaking me out.
Sunscreen and Sunglasses: The sun can be brutal out there on the water, so make sure to protect your skin and your eyes. Those things are kind of important.
Knife: Generally blades under four inches are allowed on board, but check before you go. The smart play would be to get a Swiss Army Knife for all the extra gadgets.
Waterproof Goggles: To keep the salt water out of your eyes and any oil floating on the surface from the ship.
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.