When doing pool floating items, I always use green sheet styrofoam, two layers stacked or the 4" thick if you can find it, with a container atop the foam. We use Tee Pee to cover the mechanics...then design up from there.
I then attach them to the sides of the pool with fishing line since it will not show, but will keep the pieces where I want them in the pool.
One thing, if you can, it's best to either design on site, or in a tub of water at the shop, cuz these things can be buggers to balance.
The Oasis sheets will NOT work.