I have used an inexpensive snuggli carrier in the water for my babies in the past. I have one that I have just for the pool. I hang in up in the shower when we are done and then it is dry and ready the next time I need it. So that might be an option for the hotel pool.
I haven't been to a disney waterpark with kids but, the others in FL have required the baby wear a coast gaurd approved PFD, which they supply if needed.
I was a nanny for a family with a 3 year old and a new set of twins and I now have a 10 month old and a 3 1/2 year old. Honestly, I really believe that water parks require 1/1 supervision until the kids are at least 6 and good swimmers. It is just too hard keep hands on the babies and eyes on a 4 year old and the risks are just to great.
All of that said, I would do the water park in the middle of the trip when the babies could use a down morning, have one parent say at the resort with the twins and the other parent enjoy the morning at the WP with the older sib. It could be a nice 1/1 time for the parent and the sib. Even with my 2 kids, our last trip I found that their needs were so much different, we often split up as a family to make everyone happy.
Good luck with your trip.