Drinks, snacks and towels are free for everybody on
Castaway Cay.
Yes, you have a "private" shower to wash your feet, but there are showers all over the island. There is no bathroom in the cabana, so you still have to mingle with the common folk there.
There are lots of shady places to sit and lots of beach to play on for free.
If you are self-reliant, mobile, and likely to be all over the island, cabanas are probably not worth the money. If you intend to be in one place and desire extra assistance, it's a great deal.
It's not totally private, you are sharing the "private" beach with the other 16 cabanas.
But you don't have to wait in line for your tubes or snorkles, the water and snacks are right there, you don't have to go get them. There are extra towels, and if you run out, someone will go get more for you. They will send a golf cart to take you to the tram (for some reason, they wouldn't take us all the way to the ship -- just to the tram station.) And it is a long walk, so if you have mobility issues that could be a BIG help.
With two babies, it was a lifesaver. If it had just been me and my two teens, I think I would have chosen to spend the money elsewhere.