When you are on a lake in a room and you choose to try to join your friends room one of two things might happen. You may get into your friends room if there is room or it will tell you the room is full and place you in a different room. Either way, it doesn't matter it will always then tell you that your boat will be moved to initial position because you've decided to move to a different room. Sorry this is just the mechanics of the game and isn't a bug.