Could you plant a tree that likes moist soil to get partial shade? I just had to pull out a red maple that had seeded itself of the edge of our pond and was growing well with roots into the muck. There are other trees you might consider which I googled- some get huge of course :Taxodium disticum - Bald Cypress
T. ascendens - Pond Cypress
Water Tupelo
Quercus bicolor - swamp white oak
betula nigra(?) - River birch
Or something rare the Glyptostrobus a Chinese relative of Taxodium and almost extinct in the wild.
Also you can plant LOTS of water lilies and cover most of the water- that does the same thing as a shade tree would.
