Lorilori,
I own at Grande Vista and the pool area is my sore spot. They have a fountain area where water sprays continuously flow up that the kids can run around in. They also have a "waterfall" in a large free form pool (not zero depth) that dumps a large amount of water continuously from about a three foot drop. There is a kiddie pool with a climbing area/pirate ship, water fountain sprays and a pool. (My kids, 7 and 10, are too big for it now. They are also great swimmers so we don't do that area.)
They have a lap pool too. They have hot tubs and a sauna. There is a small playground on the beach. (There are also more "quiet pools" sprinkled around the resort.)
What Grande Vista does have is an indoor play area that is awesome. My kids love it. It is like a smaller version of the hamster rat runs that they have at Bonkers or some Chuckie Cheeses. (It has a slide, a swing across a chasm, in addition to the tunnels, etc.) They also have a great "kids night out" program where the kids can play in that area, watch a movie, eat pizza, make science experiments or be a pirate, depending on the theme of the night. (You can also go watch the kids, meet other parents and talk, and read the paper while they run themselves ragged.)
I have stayed at Universal's Portofino, Disney's Yacht & Beach Club, etc. The Grande Vista pool just doesn't compare. I'm hoping that with the addition of the newer buildings that they will create a pool with a lazy river, etc. if they are worried about liabilities with a slide.
I'm sorry, I have never been to Horizons (but I wish they would add some of those things that I have seen pictures of to the Grande Vista!)
Hope this helps,
Katherine