There are no All-Inclusive beachfront locations in Cayman with a true sand beach. Spanish Bay Reef Resort does offer day passes in conjunction with shore excursions offered by certain cruise lines. You could contact them to see if this can be set up independent of your cruise ship, or perhaps your ship offers this shore excursion option. The shoreline at Spanish Bay is ironshore... definitely not sand.
VERY heavy import duties (taxes) are assessed on alcohol in the Cayman Islands. As such, all-inclusive options are not widespread here. There are many snorkel tour operators, but none I've encountered in my four years here that offer an all-inclusive option.