As reference and from experience (hard knocks) the wild native white waer lilies (rarely some with light pink hue) will easily spread into and completely cover your 20X40 ft cove in about 3-4 yrs. From there they will fairly rapidly spread into the main body of water to depths of 8ft deep. Spatter dock doesn't spread quite as fast but after awhile will encroach into depths of 9'-10ft. The lilies seem to have a clearing affect on the water facilitating their spread.
There are some dwarf hybrid hardy water lilies that will only grow to 2'-3' deep and spread very slowly. Those or the other small varieties would be best for the cove. Use the medium and large varieties in the main pond where depth spread can be from 3'-6'ft deep.
If you want to read more about using hardy hybrid water lilies for ponds buy the back issue of pond boss magazine Jul-Aug 2010 for the article (Hybrid Water Lilies Part Two by Bill Cody). It provides some background, benefits, blooming, fragrances, burning of some varieties, rhizome features, choosing varieties, planting, transplanting and dividing. Part One of the story (May-June) talks about all types of lily like plants in ponds many of them not being suitable for ponds.
http://www.pondboss.com/store.asp?c=9Pond Boss Mar-April 2010 is part one and discusses water lily look-a-likes (uaually problem types), tropical water lilies, and some features of hardy hybrid lilies.
