Features to consider when buying a snorkel are: The snorkels
diameter; whether you want a purge valve; if it has a smooth
bore and adjustability.
Snorkels with a small diameter have a greater breathing
resistance. It can be like trying to suck air through a straw.
Swimming on the surface while wearing full dive gear can be a
workout, and using a small bore snorkel can make you feel like
you're not getting enough air.
Purge valves can make the snorkel easier to clear, but also
increase the price. They work because water seeks its own level.
When you surface after a dive, the water in the snorkel above
water level drains through the purge. When you clear the
snorkel, you will have to blow out less water..
A smooth bore snorkel will breath and clear easier.