Plants, especially greedy root feeders like Tiger Lillies, will do much better if you use some type of soil in the pot. It is true that aquatic plants can get many of their nutrients from the water. However, the substrate can be a much more concentrated source of nutrients, especially iron and phosphates.
I use either garden soil or garden soil layered with a thin layer of potting soil. I wouldn't mix the two.
I'd start out with a small pot. A too heavy dose of soil can cause problems (anaerobic, metal toxicity, etc) if the plant is small and weak or the lighting is poor, etc, etc.
Potting plants in soil doesn't always work, but if you get the right combination, you'll see what a Tiger Lilly can do when its "on steroids".