Interesting project but one that will take some minor engineering I am afraid. You apparently will have to build a structure that withstand the elements and in a 13K acre lake you can get some pretty rough conditions. I have built small floating wetlands in ponds using pvc frames but they can flip over even in a protected area with the right wind. Do you want to do this in a protected area near your dock or are you talking about placing these in numerous areas in open water throughout the lake. If the later is the case, you are going to have to put a lot of thought and effort into it. Also, you may want to consider using emergent wetland vegetation rather than water lilies. I can envision a floating frame with some sort of bottom that can hold soil but is porous to allow water to seep in. This could be a plastic pond liner with holes punched in the bottom. The end result would be the equivalent of a floating marsh. If you can get it started it may take a life of its own but if it breaks up in bad weather you might end up with a ship wreck on your hands to clean up. Give a little more detail on what you are planning.