You're supposed to use the same sense you use when drinking and driving.
You don't do it. If somehow you do, you only drink 1-2.
Everybody that isn't captaining/driving the boat, can drink of course, and when you're outside you should always have more then beer in the cooler. Maybe some soda or lemonade or something, and definitely lots of water.
Lots of people die, hell I remember one year, I think 1993, 2-3 spring training players, Tim Crews, Bob Ojeda, and Steve Olin, died when they got into a boat accident. I think it was Saguaro lake, but might have been one of the others around here.
I think one guy survived, but didn't play in the majors again. Something like that.
Drinking around water is really risky, if done stupidly. Also since many people driving boats later have to drive their cars hauling tons of gear behind them they are just a train wreck waiting to happen.