Sounds like many here are uncomfortable that alcohol is allowed out on the water at all for anyone.
Based on my guess...if in the over 50 crowd, we have probably flown in an airplane that was flown by one or both pilots under the influence at some point in our lives...doesn't make it right but based on fatal airline accident reports...it seemed to me alcohol was a lot less of a risk than many other factors out there.
None of what I have been posting justifies drinking and boating ...but most of what's been posted against it doesn't justify the "backlash" society has against it either and the thoughts of controlling it (especially in the same breath many here use in "losing freedoms".
Drunk and drinking as pointed out already go hand in hand...but are NOT synonymous....
I know one towboat business is zero tolerance for their captains.... even though federal law doesn't quite put it that way...I would guess all the companies are so don't worry.
I have to disagree with one part of your logic. The fact that there are other risks that may sometimes be equal or greater doesn't change anything regarding alcohol. That's like saying it's fine to use cocaine because meth is worse.
While I agree that the impairment at low consumption of alcohol might be minimum, that does not mean there isn't some lessening of skills and reaction. I agree sleep deprivation and other things may also cause impairment.
I accept the laws as they are but not the lack of enforcement on the roads or water. Therefore, I accept that driving ones on boat if they are under the legal limit, then that's their choice. Over and they're in violation of an established law regarding alcohol whether they agree with that or not.
As to my own boat, I go with a more commercial philosophy and I exclude any alcohol while operating. Incidentally, I also don't allow operation on narcotic pain killers either or while consuming codeine cough syrup every two hours. I apply these rules to myself as well.
I restrict the alcohol consumption of guests simply as a safety matter. Look at the percentage of drownings alcohol related. I don't have a zero consumption rule for guests but I do have a two drink limit. At our home, we don't have that. The difference is obvious. You can't fall overboard in our living room. And, yes, rather than try to gauge level of intoxication, we have one rule for drinking at our home. You surrender your car keys and don't drive that night. You're welcome to spend the night or we'll get you a taxi or limo or have a non drinker take you home. Simple, our home, our rules. All our guests know and accept.
As to operation of commercial vehicles, land or water or air, we have in addition to the laws and regulations, insurance requirements and liability requirements. If as an employer I readily allow employees to drink and drive my vehicles then in the event they wreck while under the influence both legally and in my mind morally more guilt attaches to the company. Better to have zero tolerance and be sure that's made clear.
So ultimately we have two sets of rules. Those established by law for boating under the influence and those each of us choose to set for our own boats.
As to what many are uncomfortable with, there is a broad range and all is acceptable for themselves and for their own boat. Many are uncomfortable with any alcohol consumption on the water. Often they have very good reason as their lives have been hurt badly by drinking problems of themselves or others, or maybe they've lost a friend or family member to a drunk driver. The only way to insure one never has a problem themselves with alcohol is to never consume any. So they take that route.