This is an unusual glitch but it's worth a try. We have no choice but to use R6 in our project as R15 creates some unusual glitches in the gameplay. This glitch happens every time.
Basically the boat here has network ownership to the server for our game purposes. When an R15 character walks, collides or jumps out of a seat on our boat whilst using terrain water, it causes some unusual physics. I'm not sure if it's the bottom joints clipping or something but I'll let these gifs give you the visual.
As you can see here we're just running on the back of this boat and jumping in and out of the seats, which unusually jerks the boat
It gets more severe with more players, the force converted from people walking and jumping on the boat makes it difficult/glitchy to drive the boat
If you need any more details I'll be happy to provide! I'll tag you @zeuxcg as I was talking to you about this yesterday