Is light send from the top of a ship to deck different when the ship is moving or not? If you drop a ball from the top downwards to the deck it will follow a rather straigth line because the ball gets also a force horizontaly because the ship is moving. That is the cause in history why people didn't thougt that the earth itself is moving.
But I think light can't be pushed so it seems that a laserbeam not exactly follows the same path if the ship is moving or not. Probably because of the short distance it will be hard to measure. But the question is of course more principle.
So does is make a difference if the ship is moving or not?
 A: Suppose the ship is moving at constant speed to the right. And consider the situation from the point of view of someone standing on board of the ship. From this person's point of view, the ship is at rest. (She will see the sea moving to the left with respect to her and the ship.) So when she sends a beam of light from the top downwards, it will arrive at the spot on the deck that is exactly below the point where she emitted it.
Now consider the situation in which you are looking at the moving ship from an island. The light should still arrive at the spot exactly below the point where the light is emitted, so from your viewpoint it will ideed look as if the light has inherited some momentum from the movement of the ship.
So from your point of view the light has some velocity in the direction in which the ship is moving. However, since the speed of light must be the same in all inertial frames of reference (this is the special theory of relativity), this means that the light must, from your point of view, have a smaller velocity in the vertical direction. Then you will probably realize that it will take longer for the light to reach the deck of the ship. This is exactly where time dilatation comes from. The person on the ship will have a different measure of time than you have when standing on the island. What a mysterious world!
