I've done it 5 times, and it's awesome! The same people have been running it for years, and they're fun & know what they're doing. It's well worth the money.
I was on my phone when I posted before, so I wanted to add (easier to type on the desk top) that the snorkeling at Castaway Cay from the shore....on your own.....really isn't all that great. Off the family beach, Disney has planted some items to look at, mostly for the kids. The sand gets churned up from too many people walking out there, visibility can be bad.
At Serenity Bay, you'll see a few interesting fish, maybe a stingray....but neither of these compare to the reef where the catamaran takes you. There are two reefs, they rarely go to the first one because it's very shallow, but it's awesome. The second one, that they go to more often, is straight out from Serenity Bay....it would be an awfully long swim to get there, but I believe it has been done.....and it's a really good reef with tons of fish.
If you see my tag about snorkeling, you'll see that I've done it all over....really....and I wouldn't waste my time just doing it from shore at CC.