haha .. I know this from my own engine. In my engine, it happens if something is pushed up and down at the same time through an object, it just adds the velocity up and thows you up or down. It could be, that the the col gone through the ground, got stuck in it and then a wave caught the boat.
You should try flipping the boat somewhere at the beach where water and sand start to contact. Make sure you stay in it and that it has enough speed left. This is my only idea :/