I understand being drunk and on a boat has been pretty lax in the past. I would guess a part of the reason is most sea goers used to be drunk regularly. It isn't like a car, you have a huge range of not running into someone. On top of that driving a sea vessel is pretty alcohol friendly in my experience, however there is a limit.
The more we populate this world the more the laws on this should be regulated. Also the more people who are not regular water goers use alcohol with boats, accidents will rise. Using a boat is a skill, using alcohol impairs that skill. If your skill of using a boat is not so good to begin with and then you add alcohol on top of that, . . .
Further if your having alcohol parties on the water, I envision a lot of very young yuppies out on their mommies and daddies boats. I say regulate this sort of activity for sure. The ones that actually use their boats responsibly will probably sail right past this sort of law because they have enough sense not to run their boats into something.
I believe that a lot of the boating accidents in recent years have been the result of the boat being operated by a drunk driver. It is because of this that a lot of the boat regulations in the ways of open containers and drinking has become tighter. I believe if you have a responsible boat operator who does not drink then I see no reason why you can not have a small party on a boat with drinking as long as everyone is safe.