Every moment you’re out there you are making decisions that effect the future of the voyage and immediate safety. Both “long term” planning (hours, days) and immediate action is badly effected by alcohol. 

 Some first or close secondhand anecdotes:

  You’re a boat owner and have hired a captain; you bring along “crew” who bring along lots of hard liquor; they drink, and then they think they’re in a "Pirates of the Caribbean" film; they get on board and start shouting orders and freeing lines at random; the boom swings wildly, people are almost badly hurt.

 You’re going up the East River in NY with the flood tide behind you and are approaching the point where you have to go east or west around Roosevelt Island; but you’re drunk, and have forgotten the lift  bridge on the east side; the result ? Don’t ask.

 It’s after hours, on the dock, and folks who are drunk are making plans for the next day that conflict with sense and sensibility; an argument, at least, is the likely result.

 Boating is not like sitting at home in a quiet, safe place, where the odds of needing your wits the next moment are slim. While it’s true you can't easily kill yourself (as you could drinking and flying a plane) you can very easily ruin a voyage or damage your boat or other people's boats; I’ve seen it done. Please folks! No booze and boats!