Put your passport in a ziplock bag or other waterproof pouch. Leave them with your other personal gear on the boat while you are snorkeling. A crew member or two will always be left on board to watch over “stuff” left by the clients.

If you are a bit paranoid, you can purchase a waterproof pouch with a lanyard that hangs around your neck and take your passports in the water with you. AquaPak and other companies make pouches in all sizes. Google “waterproof pouches” for dozens of links.