It's not true that areas unsafe for swimming are not marked - they are; moreover, there's both infrastructure in place to increase safety (e.g. lifeguards) and a significant amount of effort put towards educating people about the dangers of things like jumping into the water in a potentially unsafe place.
But that's all beside the point. Laws and rules do not exist in vacuum, and humans are not spherical cows of uniform density. Time and again history has proven that most people can handle exposure to water safely, while they can't handle being exposed to hard drugs. You can blame this on individual stupidity, but people don't have perfectly free will, and if this stupidity predictably touches big fractions of a population, it's time to mitigate it.
Neither do most drugs, especially most illegal drugs.
Many legal drugs do (the most addictive of all being nicotine), but that also doesn't matter and is besides the point.
These are health issues, not criminal issues.
Addicts suffering from water withdrawl often drink amounts that are unsafe for their health which is why marathon runners have to be given water adulterated with mind altering metals like sodium and highly toxic chlorine to make it safe for them to drink.
It's kind of insane that water addiction would drive people to ingest water in such vast amounts that you'd have to add chlorine and sodium to make it safer.
If you think places unsafe for swimming are marked I would hazard a guess that you haven't spent much time in the outdoors.
It's not true that products unsafe for smoking are not marked - they are; moreover, there's both infrastructure in place to increase safety (e.g. physicians and filters) and a significant amount of effort put towards educating people about the dangers of things like using tobacco in a potentially unsafe manner.
I believe that time and history has proven that prohibition solves little, where infrastructure to increase safety and effort put towards educating people results in "less harm" -- a much better outcome for all. Some people will make a harmful choice (e.g. heavy smoking, fast food diet, sedentary lifestyle, using chainsaws alone), but society as a whole should not be punished for the choices of the few.
We talk about food addiction when people eat way too mcuh, not when people eat a proper amount to survive.