Riding a bike drunk is tantamount to drunk in public. Depending on the level of incapacitance, one can be in danger of hurting themselves, if not others. This is a chargeable offence whether disturbance is involved or not.
On the canoe deal I can see comparative negligence being considered, but that's going to be tricky. Depending on testimony, perhaps someone can be shown to have known better and there was inciting to engage in a dangerous activity. Could have been a 3rd party doing the encouraging. Or it was pure innocent stupidity for which there is no charge, unfortunately.
Put the same two drunk idiots in a vehicle on private property and it's going to court. How is that so very different? Just for grins consider a tandem bike where one was steering even though the other may have caused an imbalance resulting in injury to either or both. Adding in intoxication makes it even more of a paradox. I think you can see the dilemma.
I'm not a lawyer but the words paradox and dilemma, along with other ambiguous words, must be a challenge to deal with regarding legal proceedings.