most non commercial "foam in a can" is open celled and absorbs water like a sponge.If you swamp your cockpit at best, the boat will weigh a lot more and stay swamped, at worst you might sink the boat. I am not sure what any chemical reaction may be with the surface material of the boat either.
If you are looking for additional flotation, bags are available for most every kayak made at a reasonable price. If you are looking to repair the hull there are ways to repair cracks and thin spots that are explained in other posts on this forum.
Bent Shaft Canoe Paddles
Touring Kayak Paddles
The Kayak Wing
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