Ok, let me put it in another way...
I caught the code of the car level and I applied to my boat. I put the tires invisible and I managed to do to work more or less. Only that I know that that is not the best idea, because it is not necessary to have four objects with physics in a boat. My idea then is to catch the same code and to be alone with an object with physics and necessary of help for that. Maybe some guru of the physics gets me to help in that?