I would have seen that the engine is set sideways until Terbo said something about it. He is very much correct. Ican already hear someone saying the piston travels back and forth that it equals itself out, just one thing to keep in mind, there is an explosion on one side, giving push making the boat turn left.
I watch your vid over and over, after repostioning your engine, you will have a very good running boat. The only other thing that you may need to do is tweek the engine to the boat.