We did the Kittiwake twice. It's by far our favorite excursion on a Caribbean cruise. The usual stops to snorkel are wonderful, as is the return to the beach on the catamaran, when they raise de sail and cut the motor. They sometimes change the places where you get to go in the water based on weather - the Kittiwake stop can be dangerous when the wind is too high - but they have alternatives.
This tour is not appropriate for someone with little swimming experience or with mobility issues. Getting on the ship from the water can be challenging.
We did the Nautilus tour, not with a snorkel add-on, and it was quite fun, but it's best suited for families with young kids. Once and done for us for that one.