Hi Rowly
You might want to try establishing positions for one tub each of:
Nymphaea Gladstonia, Nymphaea Richardsonii, and Nymphaea Alba... these are largest strong growing hardy white water lilies to be found, perhaps start a couple of crowns of each type on a tub...
It may be one variety copes better than the others, then you could better choose a variety for planting the position with numbers, start them as shallow as you can rather than dunking them in the deep end...
5' deep is quite drastic for any water lily, especially if there are fish around that might take to clipping the foliage and chobbling on the roots. If you start them off in 3'x1' deep tubs with a clay and soil mix, that will give the waterlilies a good chance of the roots establishing feeder roots, I suspect the water motion will shred roots if they are started in a lot of gravel then subjected to periods of choppy waters...
If you start a tub off at water level, standing on milk crates, you can boat over the soil mix to fill the tubs, then set the rhisomes in place and with a little help kick the supporting crate from under the heavy tub and try to drop it gently into position, fortunately a soil heavy tub is much less weight to maneuver when its in water...
Planting directly into the lake bed may be regretable in deep water, if you can't reach the rhisomes to thin them out in future years, while a rambling rhisome growing off the edge of a lily tub is far easier to reach and snip off to make new positions... You can probably crop lily tubs very easy every three years for spare crowns... embedded in deep water on the lake bottom where you can't reach them is shall we say, creating a difficult task...
There's a strong growing big pink water lily, marliacea carnea... that might be of interest. Attraction is a strong grower, it seems the white that you tried may not have been of a robust enough growing habit for the position.
From the sound of the scale of the project, fertilizing the waterlilies is going to be quite academic, all it will do is dissolve off to add to the general fertility load of the lake.... what really will make a difference is starting the crowns off in soil which is heavy and free of any stones or pebbles, the feeder roots on water lilies like a good soft heavy goo, clay is going to make a difference, its loaded with trace minerals.
To give you some idea of the difference cropping waterlilies in tubs or rooted into a lakebed, imagine one knife cut neatly severing one rhisome poking over the edge of the tub, at about an arms length, underwater. Compare that, to trying to yank out a rhisome one foot thick gripping a lake bed every inch of its length by 3' long tether roots... bear in mind they are out of arms reach, under water....
By the sound of the scale of your project it is going to be quite outstanding....
