Every now and then a movie fugitive nips into a river and evades detection and certain death by breaking off a hollow reed and hiding underwater while breathing through it. Hollow reeds may be cheap, but they make rotten snorkels. At it's most basic, all a snorkel is is a hollow tube. But even this simple concept has been developed to offer specific benefits for different needs. Over the next few paragraphs we'll provide a thumbnail guide to choosing your snorkel. Early gogglers as they were titled, didn't have snorkels. They would swim along the surface looking down through adapted welding goggles. When they wanted to breathe, they'd have to lift their mouths clear of the water. As conditioned swimmers, this would have been an inconvenience they could have lived with - until someone thought of inventing the snorkel - or at least adapting something: snorkels were already used on submarines.

Using snorkels offers two main benefits. Firstly, you have an uninterrupted view of the underwater world. You're not pausing every few seconds to lift your head to breathe. It's also less tiring because your head weighs around 8 kg and that takes a lot of energy to support.

There are other advantages as well. Fish are often disturbed by jerky movements. Smooth movements are less aggressive. Lifting your head to breathe is more disturbing to marine life than lying prone and just breathing quietly through your snorkel.

Here we're really looking at the kind of tube you might use if you're not a trained snorkeler and just want to cruise in the shallows on holiday. If this is what you'll be doing, you'll probably spend most of your time at the surface. The downside to snorkels is that if the tip goes under the water, it floods. This can happen unexpectedly if you lean down to look at something, or the wake from a passing boat passes over you, or if you tilt your head acutely back or to one side.

The most basic snorkels are literally just an open-topped tube at one end connected to a mouthpiece at the other. So water going into the tube runs down to a water trap in the mouthpiece where it collects. When you inhale, you inhale water. With a little practice, it's easy to blow this water out of the top of the snorkel - it's like firing a pea shooter. But it does take time to learn and the technique does tend to depend on you having air in your lungs. If water enters the tube by surprise, then you normally only notice it after you've exhaled and this makes it much harder to clear the tube. You can always lift your head clear of the water, spit the mouthpiece out and drain it, but it's a hassle.

Instead of using a simple tube, an alternative snorkel design adds drain valves. These route any water entering the tube towards one way valves located near the mouthpiece. A very soft exhalation will clear any remaining water and allow you to breathe normally again. To help prevent occasional and inevitable water splashes finding their way into the breathing tube, splash guards are sometimes fitted to the tip of the snorkel. Water is diverted through a drain onto the outside of the snorkel. These kind of features make snorkelling for less experienced people much more comfortable.

Many scuba divers carry snorkels, without intending to actually duck below the surface. Sometimes they are used to save air, especially on shore dives, by letting the diver snorkel out to the actual site on the surface. Other times they are carried as a safety aid in case the diver finds himself on the surface and out of air. This can happen if the diver surfaces some way away from the boat and has to wait for a pick up or gets lost of even left behind (see Dead Calm for an investigative feature on dive boat incidents). Breathing through a snorkel can be much easier in a short chop than breathing through your mouth. For a diver on the surface having a snorkel also makes it easier to swim on their front while still wearing their scuba set. If the diver abandons the set then, again, a snorkel makes swimming prone much easier. Although a diver can swim on his back, and breathe easily without using a snorkel at all, it makes seeing where you are going and holding a course difficult.

Because the needs of many scuba divers so closely tallies with that of casual snorkelers, a self draining snorkel can be a good choice. One feature some scuba divers prefer is a flexi design. Flexi snorkels place the mouthpiece on the end of a corrugated and flexible section of barrel. This lets the mouthpiece hang down when it isn't being used. If you wear your snorkel on your mask strap this can prevent it catching on your regulator mouthpiece. With traditional snorkels swiveling the mouthpiece backwards, towards your ear also works well. It's usual to place the snorkel on the left so it does not catch on your regulator hose which normally leads over your right shoulder.

For a scuba diver a snorkel that isn't being used can be an encumbrance. If it is attached to your mask strap it can easily get caught up if you swim into a wreck, cave or kelp and dislodge your mask. Some divers stuff the snorkel under their knife straps so it sits flat against their calf. It can still catch on things, but you won't lose your mask. Some snorkels are designed to fold and can be stored inside your BCD pocket until actually needed. Cave divers usually leave snorkels well alone. They can easily snag guidelines. Snorkels have little value inside a water filled tunnel. If you plan to be duck diving down to get closer to the action, a simple snorkel tube is usually your best option. Adding valves and water traps usually increases the dead air space in the tube and sharp angles can interfere with air flow. For serious snorkelers or freedivers, there's a penalty to be paid in terms of breathing efficiency. Freedivers need to be able to inhale and exhale very deeply and maximise the amount of oxygen inspired and carbon dioxide expired. Valves and bends impair this by adding dead air spaces. The greater bulk also adds drag. For freedivers the drag is an impairment and can increase vibration against their head from the tube, which can be irritating and send out vibrations that can spook shyer fish. There are a few different snorkel types to choose from. The J-type is classic and works very well. Wrap arounds place the tube around the side of your face and the tip of the barrel over the centre of your head where it is less likely to get dipped underwater inadvertently.

It's sensible to choose a snorkel with a semi-rigid barrel. This means the tube has a bit of give. You should be looking where you are going as you ascend, but if you do accidentally collide with someone, then the impact is lessened for your victim and less shock is transmitted to your own teeth.

Choosing the right mouthpiece is very important. A mouthpiece needs to be comfortable or it is going to make using your snorkel very unpleasant. Modern mouthpieces on quality snorkels are usually made from soft plastics or rubber, like silicone. Harder mouthpieces, sometimes found on cheap equipment, can abrade your gums and the inside of your cheeks and lips. Silicones used in diving are also usually hypoallergenic to help prevent reactions to sensitive skins.

Mouthpieces are often offered in small and standard sizes. Some snorkels have standard regulator mouthpieces that can be removed and replaced with different types of mouthpieces. A number of specialist manufacturers offer modified mouthpieces that are claimed to offer greater comfort. Options can include bite pieces that extend further back, mouthpieces whose bite tabs are made from thermoplastic and can be moulded to fit your own teeth pattern and gum shield patterned versions. These snorkels also allow you to replace a damaged mouthpiece very inexpensively without having to shell out for a new snorkel.

It's useful if your snorkel has joints near the mouthpiece that will let the mouthpiece swivel. A simple tube without mouthpiece adjustment can end up fighting with your mouth - and it will be your mouth that loses. Swivels let you adjust both the angle of the tube for best efficiency and compensate by adjusting the angle of your mouthpiece for your comfort. Snorkels have an attachment for securing them to your mask, although you can slip them under your mask strap. Securing them to the outside of your strap makes it less likely you'll lose your snorkel and less likely that the snorkel will lift your mask slightly and create a leak. Most snorkels have a quick-release clip so that you can quickly and easily detach your mask and place it inside a protective case for safe keeping after your dive. Snorkels aren't the sexiest items in your gear bag, but a little care in their selection still pays off. It's why we've moved on from reeds.