There should be a second/third in command on duty when the captain is drinking or sleeping. (and the captain should be off duty long enough for whatever he was drinking to wear off - my understanding in ships often run 4 hour shifts so there may not be enough time but sometimes the second will take a longer shift to allow the captain time to celebrate.)

Of course no good captain would leave the ship in control of a new second in command in hard conditions. However sometimes the second in command would be captain years ago if there was need for a captain but there isn't, and then there is flexibility.