Maybe multiplying it by -2 would work better I’m not sure 
You can also try: Stopping the player’s impulse if the player is walking into a wall on the boat. So that on event hit you check the dot product between the player’s normalized velocity and the normal of the boat wall and if it is at a certain angle you won’t apply velocity to the player. You might need to figure out how to ignore the floor of the boat tho
Honestly, this option in Unity sounds great, I don’t think UE4 has this as I have faced a very similar problem and no one could help me either.