Instead of using sheet rock, you can have Ag lime dumped in it - Is the pond where you can get a dump truck or trailer up to it?
I had my soil tested by taking samples and sending them off to the local county ag extension office and they came back with PH of 5.4 and rule of thumb is that it takes 1 ton of lime per acre to raise PH 1 point; so I started by going to Lowe's and buying 40lbs sacks from the garden department and tossing them into my first pond out of the back of my truck and after doing that for a half pallet worth of sacks I said to hell with this and went the easier way.
so I had a local contractor go out to a rock pit with his dump truck for a load of lime(10 tons) and dump piles of lime around the banks for a couple of different ponds and then take a back hoe and use the loader bucket to pitch the lime into the ponds around the banks and he then used the boom bucket to stir the water up and spread the lime around..... Worked great and since I was having other work done it only cost me an extra $500 total for the lime, trucking & backhoe
Having said that you could do the same thing on a smaller scale with a rented dump trailer if you find a place that sells bulk Ag lime(powered lime stone) and back the trailer up to the edge of the pond and use a sump pump or gas powered trash water pump with a hose and spray nozzle to spray the lime out of the back of the trailer..... If its a large pond you can use the barge method like in this videohttps://www.youtube.com/watch?v=LXH5JtEfOx4