Another approach might be to do 2 concrete boxes, the outer box with drainage holes to maintain a constant water level. The inner box, spaced five or more feet from the outer box is the foundation for the boathouse. The space between the boxes is rock filled not unlike a crib. Anchoring both boxes to the bottom with drilled rod removes the threat of creep. piling rock around the outer box like rip rap helps protect this protector box from ice. The only unforseen is the massive flood which carries the boathouse downstream. This box in a box technique has been used in tidal waters in Europe for a long time.
What does a search for boathouse foundations yield ?
The St Lawrence is a cold water envirionment to work in but the fishing is great. Ever put a muskie on the barbie ?