In even just marginal conditions the wild white water lily and spatterdock will easily grow into 7 to 9 ft of water depth. As they spread they tend to cause the water to become clearer, (competing against phytoplankton) and thus light penetrates deeper and they grow deeper, sometimes as deep as 10ft. If one wants slow, shallow growing water lilies choose the types that are labeled dwarf or miniature usu 1-3ft deep. Next larger sized group is called small spread. The medium and large spread types of hardy hybrid varieties grow the deepest and spread the quickest some up to 6'-8' deep; it all depends on variety of hybrid lily. Hybrid lilies generally do not produce fertile seeds, although there are exceptions. If you want to know all about details of water lilies for ponds get the two back issues of Pond Boss magazine and read my two articles about water lilies for ponds.
Mar-Apr 2010 WATER LILIES IN PONDS? Part 1. Cody discuses features of hybrid water lilies and four types of look alike species. Control methods discussed.
Jul-Aug 2010 HYBRID WATER LILIES. Part 2. Cody discusses the types of hybrid water lilies, size groups, leaf and depth spread, planting, dividing and transplanting methods.
There is also a fair amount of discussion topics in the PBoss Forum Archives about water lilies and similar looking plants.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=110943#Post110943 
