If soldiers and siege machines are far from the river, and then order them to attack the river’s boat, they just stop and do nothing. Siege engines often fail to attack structures on the other side of the river (occasionally they can, if they are close enough).
To add, the river is very narrow, and the archers can stand on the edge of the river and attack the boat. The current situation is that they have to walk very close to the boat before they can attack.