We went to the Castaway waterpark area this past June using the evening passes (didn't need a room as we live 2 hours away, and drove home after). We were already going to be in the area for the African Safari, so we thought we'd check it out.
I had a family member who just raved about Castaway, how awesome it was, and she couldn't believe I never took my girls there. We were just
not impressed. I am so glad we only got the $15 passes to try it out. My girls at the time were 12, 14, 16 and 18 and while we made the most of it, and did have some fun, I can not fathom paying their room rates for that experience.
I wasn't expecting it to be anywhere close to a Disney experience, but I was certainly expecting better than what it was. There are only a few "exciting" things for teens to start with, and one of the slides was shut down the whole night.
I will say that we did very much enjoy the water coaster ~ we alternated between that and the wave pool pretty much the whole night.
My niece (6 at the time), seemed to enjoy it the most, and I would say it is definitely better for the younger set.
My now 17 year old has been to Kalahari a few times and she says it is much better, so that will be our next non-Disney trip, I think!