This is true for commercial ships too. Drug use (both alcohol and various illegal drugs) is something humans turn to when they are bored, and life aboard a ship can be very boring. If it gets exciting again while you're impaired that is likely to go very badly wrong very quickly.

Airline pilots have sometimes been caught, most flights just aren't long enough for you to be sober at both ends yet drunk during the flight - but for the crew of a tramp freighter so long as they sleep it off before they come into port nobody will know - unless there's an accident and suddenly everything is happening with unseemly haste.