In real life maritime rules, they have lights on the ships to help identify direction to avoid collision.
Port side (left) has a red light
Starboard side (right) has a green light
Stern (rear) has a white (or yellow you might say) light.
The view from another ship is the important thing. If the other ship sees your green light, they can go. If they see your red light, they have to make way for you. And you see the opposite, of course, so you know what to do too.
Just a note, I think automobile traffic lights must have used this to set up their color system. Red-stop, green-go, yellow-caution.
Anyways, light add-ons like this would be cool too.
Also, having things on deck that show to others would be cool, mainly containers. Crates, barrels. Also ropes, nets, etc. Maybe this should be done as add-ons, with storage capabilities where appropriate. Flags would be great! Would be very cool to see more style on ship decks.