Perhaps the simplest bit of kit around, the snorkel allows us to breathe with our faces in the water.
There's not much to them but here are a few guidelines:
Stay away from purge valve snorkels. They are noisy, usually, don't work and the valves are potential failure points.
Make sure you get a nice, soft, silicone mouthpiece. Rubber mouthpieces can begin to feel like they're covered in sand paper after a few hours.
Get one with a flexible, silicone tube. This is much less likely to get tangled in floatlines etc.
Most snorkels do come with a keeper to attach it to your mask strap. It is best however to just tuck the snorkel under the mask strap. That way when you spit the snorkel out to dive (you should spit the snorkel out when you dive as this means it floods straight away without 'glug-glugging' or releasing bubbles as you descend) it will sit tight against your head rather than spin around like it does when you use a keeper.
If you find that you get a lot of water coming in the top of the snorkel you should experiment with where you have it sitting.
SHOP SNORKELS HERE