our first cruise we did atlantis (by booking a room at the comfort inn so that we could use the slides)..
it was ok....but not at all worth the crazy amount it cost...
by the way, i booked it on our own because back then the swim slides weren't offered via disney...so the only way was on your own....had i had the choice, i would have used disney for sure...going on your own is a hassle....but again...we thought the atlantis 'water park' was only so so....
much more fun was our more recent excursion to blue lagoon island for the dolphin swim.....i don't know if that's an option on your cruise, but if so, it's a nice alternative to atlantis...
i believe there are usually 3 blue lagoon options - the dolphin swim, the dolphin encounter (no need to swim), or just the beach...keep in mind the boat ride to blue lagoon island is something like 30 minutes....