Here's the thing: keeping and bearing arms is not inherently dangerous. The anti's will always point to the imaginary piles of dead children as proof that people need to be safety-trained to have guns.
SCUBA diving, just like driving a car, carries inherent risk whenever someone does it, and requires a skill set to continue doing it safely. The same is not true of owning and carrying a gun. That is to say, if you strap on SCUBA gear (I have no clue about anything with SCUBA, TBH) and jump in the water, bad things are gonna happen sooner rather than later if you don't know what to do... running out of air, diving too deep or surfacing too quickly, etc. Just like when you get into a car and drive on the road, if you have no idea what you're doing, it will be due exclusively to the skill of other drivers that you avoid a collision. But if you set a gun on a table, or stick it on your hip, you can keep doing that 'til the cows come home, and no unsafe conditions will occur. Now when you fire the gun, that's a different story entirely... and this is why in incorporated areas, there is generally a prohibition on discharging firearms unless you have a damn good reason.
The true error in the author's line of thinking is that if there are two extremes, then both must be wrong. This isn't true. The vast, vast majority of gunshot injuries and gun deaths in this country are not due to pure negligence, but rather due to criminal intent or poor judgement. I'm not going to bother calculating it out again, but if a gun is accidentally fired in a random direction, there's very little chance of it hitting a person. The "accidental" shootings generally require a gun's being pointed at someone in the mistaken belief that it can't be discharged. The other shootings occur because a criminal wants to commit a crime. In either scenario, the problem is judgement, not a lack of skills. There's no safe way to jokingly point a gun at someone and pull the trigger. Long story short, neither governmental nor private involuntary training is going to prevent or greatly reduce gun deaths, just as driver training doesn't prevent Soccer Mom from doing 55 through a 25 zone outside of a school at dismissal time while talking on a cell phone.
Then there's the issue that the analogy is also false because SCUBA diving is a luxury, whereas gun ownership is often a necessity. Most people won't need to SCUBA dive to protect their lives and the lives of loved ones, and won't need to SCUBA dive to overthrow an oppressive government.