If a second is long enough to be deadly, then surely they should never, ever be around water at all, no swimming, no paddling in the sea let alone a bath. Ban it all.
Yes, and let's ban alcohol too because a few people die of alcohol poisoning.
Or you could stop with the hyperbole and look at the facts. 
Delayed drowning is rare, but it happens. That's why it's important to watch your child and know they inhaled water so you know what signs to watch out for. It's important to be near by, because if you're more than a second away, you're enough seconds away to make it fatal. If those children you saw slip under didn't come right back up, you'd bet that an adult would be quickly whipping them up. That's the entire point.
A parent that is a second away is likely sitting on the toilet reading a book while their child plays. Any further and you've just added time to the clock with how long they've been under.
Arguing it doesn't take a second, still doesn't justify leaving your child unattended. It doesn't take a second, it takes a few, and if you're a room away, how many seconds does it take to get to them? That's right a few.