Because it partakes of the motion of the boat, and moves as far as the boat moves.
When a stone thrown from a car is aimed at a post, where will it strike?
The stone will strike ahead of the post.
How far ahead of the post will the stone strike?
It will strike as far ahead as the car moves while the stone is going to the post.
Where must we aim so as to hit a bird that is flying?
We must aim ahead of the bird.
Why must we aim ahead of the bird?
Because the bird will move a short distance through the air while the shot is reaching it.
When a body in motion strikes one at rest, is the force felt alike by both bodies?