Some people book a room at the Comfort Suites (which has "rights" to the use of the Atlantis waterslide, lazy river etc).
But I believe the cost is just about the same if you book it through Carnival (last time I checked)...unless you catch a special on the Comfort Suites room. Just verify that Atlantis access comes with the room (I believe for up to 4 people) before you book it if you go that route.
And this advice is just "mother hen" talking...take the land taxis and not the water taxis. They really pack those water taxis and if you are seated in the "interior area" and that thing flips.....there would not be a good result.