A snorkel is a simple but essential piece of gear. It's very difficult to freedive for an extended period of time without a snorkel, so almost any snorkel will be a big improvement. But when you've gotten used to a good snorkel having the right features, using a bad snorkel is no longer an option. So as with all our spearfishing gear the snorkel receives its share of attention. Following is an in depth write up about what makes a good spearfishing snorkel.
If you want to skip straight to the conclusion, this is a good snorkel Speardiver Freediving Snorkel.
And this is one of the worst snorkels.
1. Flexible vs. contoured
A freediving snorkel should be contoured rather than flexible. The contoured snorkel will help you effortlessly keep the mouth piece in your mouth. The bottom/corrugated half of the flexible snorkel is straight and to put the mouth piece in your mouth you have to flex it. The tube then wants to go back to straight at the same time pulling the mouth piece out of the mouth. This results in a slight effort (pursing your lips) that has to be maintained to keep the mouth piece of a flexible snorkel in the mouth. Flexible snorkels are for scuba divers in order to keep the mouth piece out of the way while they're sucking on the regulator.
A contoured snorkel should have a moderate curve. If the curve is exaggerated the end of the snorkel will not be sufficiently raised above the water.
Flexible snorkel (bad).
Contoured snorkel with moderate curve (good).
2. Dry snorkels
The snorkel has to be top light. There are snorkels that proclaim themselves to be dry, meaning they don't allow water to come in through the top of the snorkel. Some actually keep water out and others are just gimmicks but either way a dry snorkel comes at the expense of having a big mass sitting a top of the snorkel. You can feel this when you're surface swimming and when you take the snorkel out of your mouth to dive, it dangles, bangs against your head, and creates drag.
Some snorkels have a very simple dry system installed, it consists of diagonally placed partitions at the top of the snorkel. I find that this system restricts air flow and requires more effort to purge the snorkel.
In conclusion any advantages gained by allowing less water to come into the snorkel are offset by making the snorkel top heavy and/or restricting air flow. A simple open ended tube is the best. What controls the amount of water entering the snorkel when freediving should be your breathing. When you build up experience freediving you'll develop a feel for when water is about to enter the snorkel and will automatically exhale rather than inhale.
Dry snorkel (bad).
Snorkel without dry system(good).
3. Purge valves
There are a variety of snorkels with purge systems. Some are much bulkier than others. The really bulky ones add significant mass to the snorkel. This is bad for the same reasons mentioned with regards to the dry snorkel system.
It has been my experience that some of the more streamlined purge systems simply don't work. Rather than push the water out with your exhale effort the purge valve allows air to escape from it leaving less air to travel up the snorkel to push the water out. Sometimes the purge valves work well when the snorkel is new but start to malfunction after some use. The best system is a simple tube with no purge valve.
Snorkel with purge valve (bad).
Snorkel without purge valve (good).
4. Tube stiffness
If you put a hard plastic snorkel under a mask strap as opposed to using a snorkel keeper it would put unnecessary pressure against the side of the head and/or ear.
Soft snorkels made of Silicone are available. The softness is demonstrated by twisting the tube. If the snorkel is too soft and you like to tuck it under the mask strap it might get compressed to the point of restricting air flow. If the snorkel is too soft and you're swimming in current the snorkel could bend impeding air flow.
I prefer a soft Silicone snorkel of moderate stiffness, one that will keep its shape under normal conditions at the same time no so hard as to leave dents in my head if it were pressed against it.
Soft snorkel made of silicone, moderate stiffness.
5. Tube bore
The bore or inner diameter of the tube is important, too small and air passage is impeded, too big and it's difficult to blow out water. I can't provide a measurement that's ideal because I have not yet used a tube that was too small. But between two otherwise equal snorkels where it is evident that one is smaller bore than the other I will choose the larger.
Larger bore ----------------- Smaller bore.
The mouth piece is where the choice becomes highly personal, but I still believe in some general guidelines.
6. Mouth piece height
Below is an example of a snorkel that is elegant and representative of freediving, but I will not use a snorkel with such a mouth piece. The distance between the mouth piece and the tube underneath is too small to get my lips around the mouth piece properly. My lips are prevented from sealing around the mouth piece by the tube underneath. I'm amazed this snorkel is even sold.
Mouth piece too close to tube (bad).
Conversely here is a mouth piece that extends too far out of the bottom of the snorkel. This allows the bottom of the snorkel to create leverage on the mouth piece. Any movement of the snorkel is amplified at the mouth piece and tends to pull the mouth piece out of the mouth, resulting in increased effort pursing the lips to keep the snorkel in the mouth. This effect is most often seen with snorkels that have a purge system.
Mouth piece too far from the tube (bad).
This is a mouth piece that I found to have ideal height.
Mouth piece a correct distance from the tube.
7. Mouth piece shape
Some mouth pieces are replaceable, they are the same size as the mouth piece on a scuba regulator. I find those to be too large. The mouth has to stay open too wide to hold on to them. Scuba divers are in the water for 30-45 minutes, we're in the water for hours, over this time the discomfort of a large mouth piece tires the mouth muscles. This kind of mouth piece is often seen on snorkels with a purge system.
Mouth piece too big (bad).
Mouth piece good size.
There are many cheap snorkels available that do not use a scuba style mouth piece, this makes them inherently more suitable for freediving. But there's one further feature I've seen that makes a mouth piece even better; just at the point where the tube joins the mouth piece the width of the the tube is reduced further. This makes the air hole in the mouth piece a little smaller but it increases the comfort of the mouth piece significantly. The mouth is able to maintain a more natural just slightly open position while effortlessly holding on to the mouth piece.
Describing all the various little nuances of a mouth piece shape will take too long. When you select a snorkel you should really put it in your mouth before buying because that's the only way you'll know how it fits you.
8. Snorkel keeper
There are guys who tuck the snorkel under the mask strap. I don't like this because it feels like it's pulling the mask out of alignment, and also the tube puts pressure on my head. The mask straps putting pressure on my ears is uncomfortable enough by itself. Also I'm so used to the snorkel being connected to the mask that I'm worried at some time I'll forget and take the mask off without holding on to the snorkel and lose the snorkel. It's convenient knowing that the two are always together. One real advantage to tucking the snorkel under the mask strap is how well it keeps the snorkel out of the way on a dive and stops it from bouncing around.
With that said fancy snorkel keepers are completely unnecessary and often put painful pressure on the head at the point of contact, the first thing I do is remove them and throw them out. The best snorkel keeper I used for years is a simple figure 8 shape that I cut out of a bicycle inner tube. My main consideration for a snorkel with regards to a snorkel keeper is how easily I can get rid of the original snorkel keeper without affecting the functionality of the snorkel. A snorkel like the one below is out of the question just because of the built in keeper.
Snorkel with built in keeper (bad).
The Speardiver Low Profile Snorkel Keeper pictured below has a very low profile, it's similar comfort wise to my old homemade bicycle inner tube snorkel keeper, and is more durable. It's the snorkel keeper I've been using for years now. At $2.50 it's a good investment.
Speardiver Low Profile Snorkel Keeper
9. Price
I've been seeing some crazy prices for snorkels. Many snorkels marketed for freediving and spearfishing are upwards of $30, some as high as $50. For a piece of gear that is best in its most simplest form those are some outrageous prices. I believe that from the retailer's standpoint there are items on which money can and should be made, while other items are more of a service to the customer. Snorkels should fall into the latter category IMO, they are fairly cheap to make once the design is right.
The snorkel pictured below has ALL the bad features that I don't want to see; flexible, small bore, dry, purge valve, large mouth piece, and a built in snorkel keeper. I'm sure it's priced accordingly.
A good snorkel that has all the desirable features I mentioned is this post is the Speardiver Freediving Snorkel. That's all I have to say about snorkels for now 
