Let me chime in on a couple of issues as a father and diver. I strongly agree with the above points about obtaining OW training and requiring flotation for children on snorkel trips.
As far as OW training goes, remember that you, as dad and dive buddy must be able to rescue both yourself and your son. And, mature or not, experienced in swimming or not, adolescents are more prone to panic than adults, and panic can kill divers (either you or your son or both.) Honing your skills and allowing him to receive full training will leave you both prepared for the unexpected. I always say, "expect the best, but prepare for the worst."
As far as flotation devices go, I'm a stickler with my kids. On snorkel trips with long survace swims, everybody in the group must carry backup flotation devices. I usually wear my bc, with it's whistle, mirror and sausage. The reason? The places where we dive require relatively long (100 yds or more) surface swims and are in areas where currents can be strong and unpredictable. Add boat traffic to the equation, and you've got a situation that requires a little more diligence than shallower, close-in snorkeling. I like having the ability to establish positive bouyancy and wait out a bad situation, and the ability to signal for help if we get into trouble. That being said, my kids are great swimmers and are considering OW training. Up to now, though, snorkeling has been the right fit for them.
Best wishes on your vacation. I hope you have a great time and see lots of interesting sites.
-Grier
