Look for lily varieties that list the amount of spread and approximate depth they grow. Smaller spread means slow growing. Large spread means deeper and faster growing. Keeping lilies in pots minimizes the absorption of nutrients from the pond and soon causes them to become pot bound and decrease in number of blooms produced. If you buy the right variety of lily you will never have to worry about controlling their spread. Lilies can easily be controlled with glyphosate based herbicides applied to the floating leaves. For listed growing depths add usually 12"-30" depending on the variety when they are growing 'free' in a dirt bottom pond. Clear water also promotes them to generally grow deeper. I have found that most 'large' yellow varieties tend to grow deeper than the other colors of large pinks, reds, and changable. However there are dwarf and small varieties of yellow. The type of rhizome also determines depth and spread area as noted in my 2 PBoss Magazine water lily articles.
Read through these topics and posts in the link from the Archives for lots of good information about water lilies in ponds. Some of the good varieties are also mentioned.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=110943#Post110943
