oh did that years ago for a Wedding when i worked in Toronto!
we did a rounded look on top of a styrofoam flat disk. the flowers came out enough that you couldn't see the disk and we had a heavy fishing line with a weight to keep them in place in the pool. the problem with that was the sloped edge of the pool they only wanted to be in a small area in the middle and in the flat shallow end.
We tried them with no weight but the wind and the movement of the water kept shoving them to the side of the pool.