Except that there are no such guarantees. It could still happen, and it does happen all the time. These are not risk-free activities, even when performed with all reasonable care. Of course, there are more and less extreme ways to engage in these activities, just as there are more and less extreme ways to use drugs.

You could, for instance, drive really recklessly, speed, or drive drunk. You're exposing yourself to more risk this way. Likewise, you could dive in more dangerous waters, and expose yourself to more risk. There are also known dangerous drug combinations, like mixing opiates and alcohol and going swimming. (In fact, mixing most any drug with alcohol is probably a bad idea, but there are some that are more dangerous than others.) And just as there are safer ways to drive or scuba dive, there are safer ways to use drugs.

Saying "we don't know what the fuck this thing does" is not really true for most drugs. Most drugs do have rather well defined effects. Of course, when you're dealing with black market drugs, the risk is greater that you won't get the drug you're expecting, or that it'll be cut with some other drug you weren't expecting, or that the dose will be different from what you expected, but there are ways to mitigate these risks. And not all illegal drugs need come from the black market. There are legal sources of illegal drugs, paradoxical though it may sound.

The other interesting thing is that not all drugs that are illegal in one place are illegal in all other places (or times, for that matter). And there are plenty of legal and yet quite dangerous drugs (like alcohol and nicotine, not to mention a plethora of prescription and even over-the-counter drugs). So the legality or illegality of a given drug need not have much to do with the its danger. In fact, the scheduling (ie. "illegalization") of some drugs has often driven people to use more dangerous legal drugs.