Wing stresses are probably a lot LESS than landing on grass because water is not as lumpy. When I land on grass, my plane bounces all over the place on rollout when it hits clumps of grass, footprints (we have cattle that time-share the runway) and general bumps in the ground. A proper water landing is a gentle joy to behold, and probably less shocking than a pavement landing. If you crash hard enough into anything, you will break your plane, but at typical landing/takeoff speeds, water is more forgiving than pavement or Mother Earth.

If anybody has EVER adjusted wing structure to compensate for weight of floats I would be shocked. Proper floats weigh very little and contribute some lift at appreciable angles of attack. Adjusting the wing structure to compensate for floats would be like having releasable wingtip sections to compensate for the loss in weight as fuel is burned off.