Ship ID is necessary, passport and/or drivers license a plus. Many of the places, like the dolphin area, offer lockers for rent in order to keep your things safe. While in Cozumel for the day, your ship pass is basically your passport and in case it is needed (for example in an accident, god bless...), the port agency will take care of Identification papers, etc....dont worry, but keep your thoughts straight also on vacation, prevention is always better. Also, sometimes operators require ID for the kids to check their ages for the different packages, so that would be a plus as well...good luck!