ORIGINAL: dlemo2000
If you want to see how much force is required to pull this boat under, try submerging a 1 gallon empty milk jug. The milk jub is about 1/3 to 1/4 the size of the boat by air volume. Toes fish could not pull it under.
But that is ignoring the self weight of the boat and its equipment. If the boat only has about 1" freeboard, then it wont take much to pull it under... and yes, the fish in the pic could quite easily do that to your boat.