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Don't be like the drowned snorkeler that I helped try to resuscitate on my vacation earlier this year. He apparently had been non-swimmer-snorkeling by wading around in shallow water near the shore, and bending down to watch the fish. He had a wonderful time... for a little while... but followed it up with some time embedded face-down in the sea floor, and a lengthy resuscitation on the beach before the ambulance finally arrived and pronounced him dead. A healthy strong young man who apparently loved the ocean, possibly could swim a little bit, and initially had companions with him... are you sure you'll do better?
Like everyone says: get swimming lessons. Then buy your own mask and snorkel, making sure they fit and don't leak. Then practice in a swimming pool until you're confident using your gear. Even if you're staying on the surface a wave could fill up your snorkel, so you need to be comfortable clearing your snorkel.
Not convinced? Time for the traditional Australian shock&sarcasm form of education...
The snorkel vest that you're planning to buy is a nice start - if things go wrong then your floating body might get found while you're still unconscious and not yet a corpse. Every minute counts! A drowned person floating with a vest isn't automatically saved, of course - see this other recent snorkeling death http://www.scubaboard.com/forums/snorkeling-freediving/293009-snorkeling-death-keys.html. But an added bonus of not burying your face in the seafloor is that your nose won't be completely blocked with sand, so mouth-to-nose will be a possibility. (Important if your rescuers include youngsters and women who have smaller mouths than yours.)
Your rescuers might appreciate it if you tow a flotation device carrying gear they'll need, like a high quality CPR face-mask with a valve, CPR instructions, and a reliable emergency communication device. Without a CPR mask your rescuers' mouths will get full of mucus, bile, foam, water, sand, etc. You might not care about traumatising people, but there's also a selfish reason to provide a mask - if the "breather" keeps stopping to dry-retch it'll throw off the timing of the person doing chest compressions.
Depending where you snorkel it may take hours for emergency services to arrive. Consider including a few long-sleeve shirts and hats so your rescuers don't get sunburnt.
If you're snorkeling off a beach that's covered in sharp coral or rocks it would be polite to include a few foam pads to protect everyone's knees. If you care about your appearance and you're planning a full recovery then you'd better wear a full wetsuit to protect you while we thump your body around on the sharp coral and rocks trying to drain out litres and litres of water (and sand, and jellyfish) to make room for air.
Be thoughtful: plan ahead and setup a trust fund to pay for psychological counseling for anyone involved in your resuscitation. Don't forget a bigger trust fund for your friends - if things go wrong not only will they have to cope with the horror of your corpse, they'll also presumably also wish they had supervised you better.
Alternatively, just go get swimming lessons.
If this post sounds a bit harsh.... that's because drowning is harsh.
If 10 non-swimmers read this thread over the next few years and decide to learn to swim before snorkeling, maybe that's one more corpse avoided. And a bunch of would-be rescuers who don't end up traumatized like me. The jellyfish. The jellyfish. Before or after? *huddles in corner, rocking*
Obviously I'm joking about bringing stuff for your rescuers. Don't tow stuff around - it would make you even more unsafe.
What is the best floatation device to use for a non-swimmer when snorkeling? I am debating whether to get a snorkel vest, water ski belt or a lifejacket. Not planning to go out too deep.