I would 'think' a captain would be expected to remain legally sober while the ship is on the seas. There are others who assume command while the captain sleeps...etc... but a captain should be available at all times in cases of emergency.
Now I have no direct knowlege of that rule, and I have no doubt that, whether or not such a rule exists, captains of various vessels DO sometimes get drunk. There is testimony that the captain of the Exxon Valdez was 'impaired' when it ran aground in Alaska.