Turn the boat over and with good light use a Dremel with a metal cutting disc and cut a slot in the end of the bolt of sufficient dipth to take a small screw driver or even a ground down Allan wrench to fit. You should be able to stop the turning and if not, use some WD-40 or similar and let it sit for a while. Sometimes the nut will release if you exert some tightening pressure first or alternatively in both directions. I have tried clamping small hardwood blocks on the wale top next to the head, but they don't seem to hold their position well. They will however help the head to stay when prepared as above. It probably would work with one or two small steel blocks . I think it is called patience. If the bolts are reusable, clean the threads well and grease a little when you reassemble. You may want to soften some slot edges as you may have a burr or two. Have fun !
Dave