It's a question of discipline and attention, really. It's hard to tell kids "OK, you have to put on your life jacket as soon as we get out of the car and keep it on until we leave the docks again" when none of the big folks are doing the same thing. (If there are kids around, I try to *do* just that, so they can see an adult is doing it. And if there are kids with us, I lead by example.)
And it helps if the kids can chose a comfortable PFD that looks pretty to them. Yes, cartoon characters, colors, all that stuff, there's style in PFDs these days. I don't believe in logos but I think I'd exploit that one.
It probably also would help--if the kids are old enough to talk and understand--to take them into the water, off the docks, and be there with them to let them see just how hard it can be to get back out. And then explain how nasty that can be when the water gets cold, so they understand *why* you want them to do this. (Without scaring them.)
24/7...well...in parts of Eatern Europe the kids do in fact wear harnesses when they are out with the parents. You'll see Mom sitting having coffee at some cafe, talking to other adults, and totally ignoring the kid on the end of an 8' lead. In the US you might get arrested for that but until the kids are old enough to understand why they can't run out in traffic, or run with scissors...
Meanwhile, perhaps a nice water rescue dog, like a Newfoundland, to watch over them on the docks?