When my kids were in their pre-teen/early teen years, they and their friends would have a great time capsizing our 17' Grumman (aluminum) canoe in relatively shallow water. They would swim underneath into the air pocket, and those of us on shore could hear their conversations underneath the hull.
Yes, a canoe full of water is heavy, so obviously one doesn't lift it out of the water, then turn it over to dump the water. In water shallow enough to stand on the bottom, two people can refloat the canoe, almost empty of water. All they have to do is rotate the canoe slowly on its long axis, with one edge clear of the water, while lifting. The hull is essentially rotated up and out of the water without lifting any of the water. If the canoe is completely upside down to start, simply rotate the hull to bring one edge up out of the water, then lift the hull slowly clear of the surface, then flip it quickly back onto the surface.
Trying to do this in water too deep for standing on the bottom and without the aid of a second canoe to provide lift would be quite difficult. The 17' Grumman weighs 75 pounds, so two adults would have to lift 38 lb each. Doing that through scissor-kicking in the water seems elusive. I wonder if two large adults could push the hull up and empty enough with a sudden upward thrust. Of course the upward thrust also would push the men totally under water. Could four do it? It would have to be done by whatever crew the canoe was carrying. Four to a canoe? Unlikely.
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