Be very careful with lotus. They can be quite hard to control and it’s quite possible they will survive fully draining a pond. They will take over, and are quite capable of chokin. Out a whole pond in a short time. The tubers grow laterally around 8-14” below the pond bottom.
As to water lily, there are two root growth types among hardy Lily’s. Some grow laterally and some grow in a cluster. The cluster root tend to be smaller, and are easier to keep from spreading out. The lateral root are known to spread over wide areas and have more spaced out pads, and typically are larger all around. Planting either in pots is a great way to do it. You just have to pull the pots up every year or two and divide out some of the roots to keep them healthy. Personally I find Colorado, Sioux, and chrysanthia to be quite nice, easy to keep, and easy to propagate. If you want to cover water surface fast, look no further than oderata gigantea. Lilypons water gardens in Adamstown MD does mail order and has a ton of varieties.
