Right Hal,
There is an explosion, the thing to keep in mind is that the explosion pushes on the cylender head just as much as it does on the piston. This is the equalizing force within the engine. this is the other thing - because the piston and the explosion are enclosed within the engine casing, it is a closed system. Bobs analogy with the weedwacker is incorrect. turning the weedwacker at high rpms is difficult, this is due to gyroscopic forces. If what Bob had said was true(and what you are saying is true with this boat), the weed wacker would try to twist out of your hand when it's throttle is gunned(you say it would twist perpendicular to the driveshaft - it will torque in your hand, but along the same axis as the driveshaft), It does not; it only tries to stay in the same orientation (like a gyroscope). An engine, unmounted, and sitting on the ground, will vibrate and 'walk' due to the vibration, but not due to the force of the piston being 'blown' one way and comming back the other(you've seen generators sitting on the ground, haven't you?).
I believe this problem is from prop walk(minor), and from engine torque. With a large engine, and a large prop, the torque will drive one side of the boat deeper into the water. with a flatbottom boat, this means the boat banks into a turn. The harder you push the engine, the harder the turn. You might try air flaps, on the cage of your boat, to counter the torque. Otherwise, the hull would need to be modified, perhaps to a rounded nose/rake so the leaning caused by the engine torque does not affect the boat as much.
Sean