Build a dam
If the water is fairly shallow, try using a piece of plywood as a water diverter. If there is any ice at all, chop a slot wide enough for the plywood to fit through. Fit the boards through the slot and into the muck, leaving enough above the ice to hold it. This will set up the ice thicker on your side of the board and give you a few extra weeks at the end of the season. If all you have is open water, the dam idea may still work, if you can figure out how to keep it standing.
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