This. Get them in the water early so that they learn to feel comfortable with it, since panic is the main cause of drowning. They should learn in a fresh water pool since after that salt water is really easy.Parents should get their kids swimming lessons at least for basic swimming. Don't depend on the blow up swim aids you can get for young children. Either have them learn to swim or wear a life preserver if the water is over their heads.
My parents took me for swimming lessons when young and then YMCA camp where there were swimming lessons every day I think. In high school I got Senior Lifesaving certified and after college NAUI and PADI certified for scuba.
In open water, be aware of currents since they can carry you quickly. Wife and I were recently snorkeling in St Lucia and Tobago to name two places with currents that would wisk you away from your boat in the spots we were in with only a brief heads-down time and we'd then had to work to get back.
An added thought is that if you're going in to try to rescue somebody, a moment spent pulling on fins and mask with snorkel would probably be time well spent, since fins greatly improve your ability to move along and clear vision in the water is always useful and the snorkel might come in handy as well.