I've had good luck pouring concrete into water as well. I dammed a creek on my property starting with bags of concrete laid as-is. I used some plastic to prevent leaks initially.
Over the winter it began seeping around the sides, despite my efforts to stop it. So, I further opened a leak to allow the water level to drop for repairs. Then I used mixed concrete to cover the dam with a layer of concrete, into which I laid a few nice rocks for a natural look. I then created a temporary berm to minimize flow around the sides and front of the dam where I wanted to add concrete, then I just poured the dry concrete where I wanted it on the loamy sides of the creek bed, right into the water and above. It takes longer to set, but it does set. Then (after that set) to seal the leak, I poured a couple of bags rather quickly into and around it and it sealed right up. Very soon the water was flowing over the top through the spillway again, and I have increased the size and depth of the pond and the volume over the top, which makes a nice waterfall into the receiving pond below.
You don't intuitively think of concrete setting up under water, but it most certainly does, right, Dennis? It's not paper mache! I would think this method would be useful for certain types of leaks on larger pond dams as well. I don't think it would have worked without the foundation of laid bags being there already, though.
Many thanks again to Bob L., Ric, and the rest of the crew.
