We have stayed at Atlantis twice during low season (i.e. not during any school breaks) and a cabanana was not necessary at all. I'm sure it would have made a nice day to spend in cushioned chairs with all the amenities provided, especially if the resort is crowded and pool chairs hard to come by. When we have traveled we have been able to take our pick of multiple chairs at any pool at any time of day. While we didn't leave valuables (like a cell phone or GoPro), we left shoes, clothes, snacks, water bottles, sunglasses on the chairs and never had any issue.
We love Atlantis - if you like waterparks, this is a ton of fun. If they aren't your style, then it wouldn't be worth the cost. The complaints I've seen tend to be along two lines: 1) Crowds - this completely depends on when you are going, and 2) cost of food - it IS expensive. When staying for a week, we cut costs by going to a grocery store on the way from the airport, but for a single lunch, it won't be bad - especially if you get something from one of the pool snack bars rather than a sit-down restaurant (which I wouldn't waste time or money on for a day trip)
You could save quite a bit of money heading there yourself and buying a day pass directly from the resort rather than through the cruise line (you will have to show your valid cruise room key). An adult day pass is $129 through Atlantis vs $195 shore excursion. It is a short cab ride and I *think* like $4 pp each way, plus maybe a toll bridge fee one direction.
Most of the slides have a 48" minimum, so not sure if your son is tall enough yet. If so, the entire park is available to enjoy.
