Now, I think of pool slide and relxation as oxymorons. To me, pool slides = crowded with noisy families. I think if you want a relaxing trip, spend time at both the pool slide pool and the quiet pools for chillin'
SSR - I though Grandstand was exceptionally relaxing (much more than Springs) The GS pool with bar was very nice and calm (but possilby more quiet as the kiddie play area was fenced off for referbishment) The main SSR pool looked crazy busy during that trip.
BCV - Not stayed there, but is is supposed to have, by far, the best slide (but I think that means busy) In visiting, I did discover at BCV you can sit on a lounge chair on a white sand beach looking at BWV, and I've never seen the beach area used much. BCV also has several internal and landscaped areas that I find exceptionally beautiful and relaxing.
AKV - Pool area has much lush tropical landscaping, cool, calm interior. Great animal viewing areas (including a public one with rocking chairs around a fire pit) The more time you relax, gazing at the savannah's the more you will enjoy the animals, because they are not just in a cage waiting for you to walk up at a moment's notice. I would say that the Jambo pool slide is not more exciting than SSR, though.
Another thing to consider, renting a boat. (At the YC if at BCV or Captain Jacks for SSR and OKW) You did mention a cruise, but nothing says relaxation more than boat time.