Waterproof Phone Case: Because it would suck to miss out on some ocean Pokemon Go action.
Personal Distress Light: Again, to help your rescuers spot you. There are several different types of these lights, but definitely bring one along with you.
Medical Sea Pack: In case you get injured or must survive days away from a doctor, you'll want to pick up a mini medical kit. Again, one that is built for ocean survival.
Food Supply: Bring enough food packets to keep you going for a few days. American Red Cross sells some great ones that are small, but loaded with what your body needs. Not sure if they have any beer packets, though...