I have got a rigidbody on my player. I had to set this to extrapolate because interpolate caused the player to move very slow bit by bit. On extrapolate the player movement is alright but the player occasionally changes z position relative to the boat. I am rotating the boat and moving it forward on z by transform. I cannot use a rigidbody to move the boat because it needs non convex mesh colliders.
Interpolate/extrapolate shouldn’t affect movement speed at all, if it does there’s something very wrong going on. These just affect how the rigidbody is rendered during frames where physics haven’t been updated, mostly useful for slo-mo effects.
A moving boat is not unlike a moving platform. Try parenting your player to the boat.
I did try parenting it to the boat. I am not sure if the movement was slow or if that was just how it appeared because it was jittery.