you didnt said more details, but, as i can see in the boat, before you control the boat, the boat seems anchored, and when you control, it becomes flexy, if thats the case, its because your character is acting like a rock in the mini boat, and the boat cannot handle the mass.
to resolve it you can:
u can apply less density to the boat and make it float
try and place a body velocity and dont let it clip in the water
place a part under the boat with welds and dont let it sink
Make your character massless when sit, and when jump go back to normal
u can try many things. Hope i helped you with your problem!