I saw a film about some professionals applying ag lime to a large pond or small lake. They used a large barge that they loaded with lime from a front-end loader. Then they drove around the pond washing it into the water using a gasoline powered water pump. They wore protective clothing and masks. I think you could build a raft or put a heavy piece of plywood between 2 aluminum jon boats and wash the gypsum or lime into the water with a small gas powered pump. Pain in the butt and probably messy but that is the penalty I usually pay to be a do-it-yourselfer.