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I still cant tell if you have a dive door on the port side, but if you do have one and the crack is across from the door +/- a few feet, it may be due to the gunwhale not having support on the side with the dive door. I believe I read that the older whalers fitted with dive doors weren't properly stiffened in that area to compensate.
Without it being braced, the starboard side would have to carry its load down the length of the boat, as well as any load aft of the door which was not connected to the gunwhale or the side of the hull. The Whaler Drive would increase the moment arm in this situation.

The result would be a crack in the area I believe you are describing. I dont think its a catastrophic failure, but it will need to be repaired somehow before it grows.

Or, you dont have a dive door and the cause could be due to something else.