I have no idea if that guy on the right is wearing pants and that is freaking me out.
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.
Waterproof Phone Case: Because it would suck to miss out on some ocean Pokemon Go action.
Locator Beacon: These small devices will send out your location to rescuers. Just make sure you get the waterproof kind.
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.
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.