I would get underwater cameras for the excursions, that way you will be able to keep your more expensive equipment dry. However, if you want to take it with you ashore, you can. I have not been to Cozumel, but I would not worry about leaving your cameras on the boat at Stingray City, there is always a crew member onboard that watches over your things. As far as your money and id, you can get a waterproof wallet or bag to keep them with you. If you want a video of what you did ahore most tour operators for Stingray City do have people doing a video of your experience that you can purchase from them. I thought it was a bit pricey $30 to $40, but much less expensive than having to replace your own camera.