Snorkels are not really necessary for diving underwater, but they are needed for use on the surface whether you are a diver or snorkeler. While they are perfect for snorkeling trips and swimming out to a dive site, they can also be used for swimming laps (with a specialized model shown below right). Depending on how often you dive and/or snorkel, you will want a more or less expensive model. Some start at under $10 and may come attached to a mask. Others will cost upwards of $20 and are sold separately. The more money you spend, the more features you will get. More experienced divers and snorkelers will probably prefer a more advanced snorkel. Serious divers will want a good snorkel, but they may not want it attached to their mask while diving since it creates drag and gets in the way.
Fit and Comfort
This may seem obvious; however, the fit of a snorkel is quite important. It should have a soft silicone mouthpiece to maximize comfort. The mouthpiece should be big enough so that you don't get jaw fatigue from biting down on it. Likewise, it should not be so big that it feels like you are chomping down on a baseball.
'''Length''' -- Choosing length is dependant on the size of your head and the angle at which you snorkel. If it's too long it will be difficult to get the adequate amount of fresh air. If it's too short you run the risk of getting water in through the top (on models that don't include splashproof technology).
'''Bore''' '''size''' -- The bore, or opening, should be large enough to easily inhale the proper amount of fresh air. Remember that the bigger it is, the easier it is to get water in and harder to get water out (on models without a purge valve). Choose the hole size according to your lung capacity. For example, a smaller bore is best for a child's snorkel.
'''Purge valves''' -- This technology is more expensive but useful for novices since it means you can clear the snorkel from below the mouth instead of shooting the water out the top of the snorkel. This is a great convenience if you don't have powerful lungs or a large lung capacity. Splash protectors can also be useful, but they have the tendancy to restrict air flow.
'''Mask Clip '''-- The mask attachmentshould be easy to adjust for divers who will be adding or removing the snorkel at surface intervals. Snorkelers can probably make due with a less advanced attachment mechanism since the snorkel will remained attached at all times.
'''Collapsible '''-- Some of the newest snorkels (particularly the Sherwood Avid and the Apollo Prestige) boast the ability to fold or collapse to a smaller size for easy storage and transport.