Sounds like someone may have placed a can of something caustic to plastic on top of it will in storage(?) I doubt there is a good fix for this issue if it is just an aesthetic problem. You would probably cause more damage trying to fix the dimple. If it were fiberglass; I would drill a hole in the center to allow a small-threaded rod to pass through. Then, I would place a small piece of chop-mat(fiberglass) and resin on each side. Cover with aluminum foil and put the rod through the hole and use a board on each side to clamp the patch with wing nuts pass through the board. This will compress the patch and remove any air voids. Let is set up for a day and remove. Then, place some masking tap on one side of the drilled hole and fill with a dab of finely diced fiberglass and resin. Let cure, remove the tap, sand the rough edges and apply a light coat of gelcoat that matches the color of the boat. Wetsand to blend and you will be ok. If this is a plastic canoe that been extruded; I am not sure you could use this approach and blend it as well.
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