I used to drink quite a lot while rafting, had the 2-stroke blender, jugs of premixed mai-tais to pass around, etc, but then I also used to drink quite a bit doing pretty much everything else. I also did a lot of Class III's and maybe a III/IV- at the most.
It was probably a worse idea shuttling or driving home than whatever I did on the river.
I don't drink much anymore, so it's really a non-issue. My coolers are too big for daytrips now.
It was after a river trip a couple of years ago that I turned into a complete drunken dick to my wife in the restaurant parking lot and that was the wakeup call that it had to stop. I didn't drink for a year, and now only have a few drinks a month.
I never have really drank while kayaking. After a beer or two, my ability to hold my breath goes from 30-60 seconds to as little as 5-10.
I have zero issue with a beer or two on a II-III (like Glenn said--runs I'm super comfortable with), and don't mind if someone on my boat has beer. I can't drink a beer fast enough on the Lochsa before it gets spilled, so I don't bother. Like TriBri1, I don't hold anyone else to my own standards--they are my own. I boat with a lot of very skilled paddlers who smoke a bowl mid-run. They're better boaters than me, and they're adults. Not my place to say a thing. It's a free country.
A cold one at the take-out sure tastes good, though.