You can usually get better prices searching excursions before you go, that said,
if for whatever reason, there are delays or some other problem, if you've booked with Disney, they'll make sure to get you on the excursion, where a third party may not wait around and you're out of luck.
Shouldn't need more than snorkel gear on castaway cay. But, MAKE SURE you and the wife ditch the kids either at the club on the island, or let them run loose and watch out for each other, so you and the Mrs can enjoy the grownups-only Serenity Bay together, at least until lunch, where they have the ribeyes and lobster on the buffet menu. The everybody buffets on the main part of the island don't serve those. Everything else is the same though. After lunch you can do the snorkeling and the little waterpark/slide areas with the kids.