I know Mariner and Navigator do not have the slides. I believe one of the Voyager class ships has them. We are booked on the Brillance and they have a slide. Check RCIs website and look at the ship plans. If there are slides, I believe they are located at the back end of the pool deck.
The Mariner and Navigator don't have waterslides, but the Adventure, Voyager, and Explorer do. I'm 15 and I didn't think the waterslide on the Adventure was that exciting, but your younger boys should love it.
As Tauruschick writes the VY, EX and AD all have slides and so do all the vessels in the Radiance class. And you might also consider the stretched and revitalized EN -with an interesting pool deck- when she is back beginning of July.