Wifey B: Things that don't float, sink when in the water. Little things you don't want to find the water do.
So, if you wear glasses, wear a band. Same with expensive sunglasses. Don't put stuff in a shirt pocket. Don't keep things like your car keys in your pocket, put them away. Don't wear rings if you value them. Insure jewelry too, requires a separate policy. Don't use cell phones and tablets where if they're dropped, they're in the water. Advise guests of all these things.
Sun! Sun burns. Bad Bad Bad.
I think as a host it's your responsibility not to let guests get burned. Keep tons of lotion on board and make sure it's used.