Caribbean, through Mexico and Central America to northern South America. It Is a Florida native. Other names include American white water lily, American water lily and alligator's bonnet. Another type found in Florida is the yellow water lily. The leaves of the water lily are nearly circular, with a slit that goes to about the center. It's sometimes confused with spatterdock, which has heart-shaped leaves and a round, yellow flower that looks like it's not quite open. Spatterdock is also a member of Nymphaeaceae.
While the leaves and flowers appear to float freely in the water, they're actually rooted to the bottom via rhyzomes, or underground stems. Water lily is commercially grown for use in water gardens. As might be expected with an aquatic plant, it does not tolerate drought.
One person's flower is another person's weed. In Washingston State, water lilies are classified as a noxious weed.
Fun fact: The young leaves, flowerheads and seeds of water lily are edible. In Egypt and the west coast of Africa, the seeds are ground into flour.
Spatterdock in the photo at left for comparison. The flowers of the plant can be seen at different stages.