There happens to be a Kijiji ad up today with some information. After reading, you might want to go buy them (It's not my ad). Good luck.
Ad ID: 154196974
"Prevents Ice Damage to Boat, Docks, Piers and Boat Houses and Pond.
The Ice-Away De-Icer’s high speed motor driven propeller brings the warmer deep water to the surface. Typically a ½hp De-Icer will clear 2.4 meters (8') in diameter of water for every foot of propeller depth. In most cases, a De-Icer placed at a 1.5 meter (5') depth will keep a 12.2 meter (40') circle of water, ice-free during extremely cold temperatures."