An item that can be added to rope and thrown at boats. It could function like other marksmanship weapons, perhaps acting faster than a sling since boats move so quickly.
Once a boat is pulled onto shore or into another boat, it is held in place until the thrower takes damage or moves.
The purpose is to make water combat viable. People in adjacent boats should be able to hit each other. People in boats should be able to tie the rope used for grappling to their boat, to free the other hands for combat items.