The rain is a two-dimensional screen, not a 3-D effect. For internal (bridge) views it is near the bottom of the layers, just above the sky. That is why it stops at the horizon.
For external views, the rain screen is the top layer so you will see it falling between you and the ships; and past the horizon.
It is not real water, so it can’t get anything wet. Adding splashes on decks (where there is not really any rain) would, as Terry said, be too expensive, and kill the frame rate.
Live with it.
