I don't have much experience with multiplayer boat stuff but when I have after someone uses the rudder and everyone has jumped of the boat it comes to a dead stop. The only way the boat moves is from tides moving boat down onto a bank which makes it slide and get inertia to slowly move away. If players start jumping and running all over the surface of the boat then it can also gain inertia from that and start moving again. If you take hold of rudder and jump off, it will again bring boat to a full stop