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I'll take #4 and #7.
Most snorkeling shore excursions include equipment or have it available for rental. I always bring my own because it fits well and I don't trust their sanitizing.
Carnival will supply beach towels. They keep count of them so make sure to return them to the Cabin Steward if you take them from the cabin or to the Towel Hut on Lido deck if you got them from there. If you don't return them they will charge your Sail & Sign account.