An extract on #ibrahimalikhan
Full face snorkel masks use an integral snorkel with separate channels for intake and exhaled gases ensuring the user is always breathing untainted fresh air whatever the respiratory effort. The main difficulty or danger is that it must fit the whole face perfectly and since no two faces are the same shape, it should be used with great care and in safe water. In the event of accidental flooding, the whole mask must be removed to continue breathing. It can only be used on the surface, or a couple of feet below since there is no way to equalise the pressure at greater depth. Trained scuba divers are likely to avoid such devices however snorkel masks are a boon for those with medical conditions that preclude taking part in SCUBA diving.
A snorkel can be useful when scuba diving as it is a safe way of swimming face down at the surface for extended periods to conserve the bottled air supply, or in an emergency situation when there is a problem with either air supply or regulator. Many dives do not require the use of a snorkel at all, and some scuba divers do not consider a snorkel a necessary or even useful piece of equipment, but the usefulness of a snorkel depends on the dive plan and the dive site. If there is no requirement to swim face down and see what is happening underwater, then a snorkel is not useful. If it is necessary to swim over heavy seaweed which can entangle the pillar valve and regulator if the diver swims face upward to get to and from the dive site, then a snorkel is useful to save breathing gas.
The greatest danger to snorkelers are inshore and leisure craft such as jet skis, speed boats and the like. A snorkeler is often submerged in the water with only the tube visible above the surface. Since these craft can ply the same areas snorkelers visit, the chance for accidental collisions exists. Sailboats and sailboards are a particular hazard as their quiet propulsion systems may not alert the snorkeler of their presence. A snorkeler may surface underneath a vessel and/or be struck by it. Few locations demarcate small craft areas from snorkeling areas, unlike that done for regular beach-bathers, with areas marked by buoys. Snorkelers may therefore choose to wear bright or highly reflective colors/outfits and/or to employ dive flags to enable easy spotting by boaters and others.
Snorkelers' backs, ankles, and rear of their thighs can be exposed to the sun for extended periods, and can burn badly (even if slightly submerged), without being noticed in time. The wearing appropriate covering such as a "rash guard" with SPF (in warmer waters), a T-shirt, a wetsuit, and especially "waterproof" sunblock will mitigate this risk.
Dehydration is another concern. Hydrating well before entering the water is highly recommended, especially if one intends to snorkel for several hours. Proper hydration also prevents cramps. Snorkelers who hyperventilate to extend sub-surface time can experience hypocapnia if they hyperventilate prior to submerging. This can in turn lead to "shallow water blackout". Snorkeling with a buddy and remaining aware of the buddy's condition at all times can help avoid these difficulties.
When snorkeling on or near coral reefs, care must be exercised to avoid contact with the delicate (and sometimes sharp or stinging) coral, and its poisonous inhabitants, usually by wearing protective gloves and being careful of one's environment. Coral scrapes and cuts often require specialized first aid treatment and potentially, emergency medical treatment to avoid infection. Booties and surf shoes are especially useful as they allow trekking over reefs exposed by low tide, to access drop-offs or deeper waters of the outer reef - this is, however, ecologically irresponsible.
Contact with coral should always be avoided, because even boulder corals are fragile.
Another safety concern is interaction and contact with the marine life during encounters. While seals and sea turtles can seem harmless and docile, they can become alarmed if approached or feel threatened. Some creatures, like moray eels, can hide in coral crevices and holes and will bite fingers when there is too much prodding going on. For these reasons, snorkeling websites often recommend an "observe but don't touch" etiquette when snorkeling.