I'm planning a cruise for my sisters. One has a green card and lives in San Diego The other one lives in Mexico and has a B1 B2 Visa. What documents do they need in order to board the ship in Long Beach, Calfornia?
Since they are not US citizens, they should have the same documents as if they went for an international flight. This means passports and green card. They would have to clear the passport control upon returning from their cruise, same as in airport (in a Long Beach terminal).
For US citizens it is possible to get away only with the birth certificate, but only if the cruise starts and ends in the same port.