The hardy hybrids rhisome might grow along the soil of a lake at the rate of three feet a year, it wouldnt be good idea to plant them deeper than in water than is comfortable to paddle in, three feet deep is a reasonable 'maximum' depth for planting any size water lily, they do flower best where the rhisome is not too cold or too dark for light to get to the crowns.
A good arrangement is to place large 'tubs' with clay and heavy loam, with about two to three feet of water over them, place say two or three crowns on a tub, in about three years time they will be well established and sending rhisomes out of the basket, which are easy to crop off when growing off a basket rim.
If planted direct in a lake bed, waterlilies would be much more difficult to control, as they take to wandering... you would find it difficult to paddle around and sort them out, or to lift the rooted rhisome, compared to starting them on tubs.
One advantage of the tubs is, it becomes possible to lift the tub up, say on to a milk crate for trimming, or even attach flotation drums and float them off to another position or to shallow water to do a little cropping
Regards, Andy
