If you know anyone with a tractor with a bucket on it I would use that. I would get as close to the water as you safely can, put a chain around that tree and attach the other end of the chain to the bucket. Lift that bucket as high as it will go and just back up. It should come right out.
If you fail at that for some reason you might be able to wade out in the pond and get a chain around the tree closer to the top. Then you might be able to spin it around and get it so its parallel to the shore line, up in real shallow water. At that point, you can put several chains or heavy ropes to it and get a couple buddies with trucks to pull it out like a team of oxen.
I don't think you will have any problems with the first method though. I think that it will come right out if you try the tractor.
