I am glad you posted this. I was thinking about this same thing because we just got some water lilies for the pond here. They came with a plastic basket/pot in which to be planted. I was thinking of just putting them in some old cracked and broken clay pots. I figure the weight of the pot would help keep them under water and "planted." Maybe that is not such a good idea after-all.
Also, for whoever would know, what would happen if we just planted our water-lilies (the hardy kind) directly in the pond? On the package it says they can be planted up to five feet deep. My mom cautioned me because she said that she and my dad had so much trouble with water-lilies taking over the entire pond. I don't think that will happen here as there is a very steep drop off in our pond. It goes from about two/three feet to fourteen feet in a matter of inches. I figure the worst that could happen is that they spread all around the edges of the pond. ANYWAY, my thinking is probably jumbled here, but why plant them in a pot at all in our situation?
Ok, that was two questions--kind of asking the same thing. LOL. I will stop writing now.