Sorry for the late response.
Did you buy the plant from the store planted in soil, then put it in gravel?
The plan was do the same as you mentioned. However the soil kept mixing in the water, so i washed off the dirt, put the roots in a pot with several holes and filled it with gravel. I have read and asked about the planting of canna lily in pond and learned that both ways are fine.
If you plan on putting the entire potted plant, then first confirm that there is no fertiliser in the soil. Then put a layer of gravel over the soil to avoid is mixing in the water, after which place it in the water. The crown shouldn't be submerged more than three inches and the less submerged the crown is ,the better.
What would be good plants? The pool/pond is outside and gets plenty of sunlight.
Taro, Papyrus(Umbrella Plant), and Monstera are some other plants which do fine in medium to high sunlight(preferably medium). I have also planted Pothos and Spider plants, however they won't do good in extreme sunlight.
Note: Taro, payrus and Monstera will rot if planted in water logged soil so it is best to plant them bare roots.
You can also plant water lotus which thrive in full sun.