Are you sure the seat isn’t Anchored and isn’t’ welded to the boat?
What happens if you wait 10-15 seconds when testing? It seems to me when testing my places that physics is one of the last things to load in.
If the boat is travelling in a straight line you could just use a PrismaticConstraint instead of a tween. That way any physics involved will move the player with the boat.
I am using TweenService, so anchored properties won’t do anything different, and the problem isn’t that the seat is not moving; they are moving the seat and the boat, but the player is not moving with the seat.
I tried to weld the player’s humanoid root part to the seat.
and also weld the seat to the boat, but nothing is happening.
Since you’re using tween service to move the boat, make a model and then parent the boat and seat to it. Then set the model’s primary part to the boat union, and in your script, tween the Model.PrimaryPart