Safety is the #1 rule of snorkeling. Never snorkel alone, don't snorkel if you can't swim and use common sense. Read more on Snorkeling Safety.
The First Time Snorkeling
The most important thing for a first time snorkeler is to get comfortable wearing the mask and breathing through the snorkel. Some people get anxious and have difficulty breathing through a snorkel while wearing a mask, so it is important to test things out in shallow water first. Many first timers have jumped into the water on a snorkeling charter only to realize they aren't comfortable wearing a mask and breathing through a tube - this often takes a few minutes to get used to. While standing in shallow water, practice putting your face below the surface and looking through the mask. You can breathe through the snorkel while looking around at the aquatic life. In shallow waters this can be a fantastic way to see many things without expending the energy of swimming.
Snorkeling in shallow water is also a good time to learn to use the equipment effectively. Although relatively simple, beginning snorkelers should know their way around their mask and snorkel, including how to clear both of water. This is an essential skill, as open waves or splashes can send water into the open end of a snorkel, and masks can develop tiny leaks during a dive. Having a mask or snorkel fill with water can be a scary experience the first time, so snorkelers should be comfortable with the process of clearing their equipment.
Clearing a snorkel is an easy process. If you find your snorkel tube clogged with water simply exhale with a strong force through your mouth, which should send the water up and out the end of the snorkel. Some snorkels come with built-in drainage valves, allowing the water to be pushed out a one-way valve. This makes it easier to push the water out if a small amount of water makes it way into the tube.
Clearing a mask is similar to clearing a snorkel, but can seem more difficult because of the reduced visibility. To clear out a mask, simply lift your head out of the water and pull forward on the front of the mask. This will open up a gap in the bottom of the mask, allowing the water to drain out. Some masks come with built-in purge valve, which serves the same role as the drain valve on a snorkel. By including a one-way valve which lets water out but does not let water in, snorkelers can clear a mask of water by simply blowing air out their nose while the mask is on. The water will be pushed out the valve, clearing the mask. Even masks without a built-in purge valve can be cleared while underwater. Simply press the top of the mask to the forehead and blow out the nose. Air will bubble into the mask, pushing the water out the bottom.
The next step in learning the basics of snorkeling is to practice while in open water, when you cannot touch the bottom. To do this you will need to be comfortable with using your snorkeling fins to stay afloat upright as well as to move around while floating face down in the water. As you swim along the surface, practice breathing evenly through your snorkel. The most common underwater kick is the basic flutter stroke. When used properly, this kick can be a very fast and efficient method of transportation in the water. As you kick, use a slow, comfortable pace and remember to keep your fins submerged in the water. You should find that a pace of about twenty kicks per minute will give you a good cruising speed through the water without too much fatigue. Breaking the water surface with your fins uses more energy and decreases the efficiency of your kicks. Keep your arms at your sides while swimming to reduce drag. Another common kick is the dolphin kick, in which both legs sweep up and down together. This kick can be more difficult to master but is a very efficient means of underwater propulsion when learned properly.
If you fee a bit more adventurous, you can practice going deeper underwater by diving below the surface. The two basic types of dives are the feet-first dive and the head-first dive. The feet-first dive is the simplest. While vertical in the water, raise your chest and arms above the surface of the water by kicking with your legs. As the weight of your body begins to pull you back down into the water, raise your arms above your head, sweeping them upward to push yourself lower. Because it is harder to sink underwater when your lungs are full of air, exhale a small amount of breath as you begin to descend. Next, pull your knees to your chest and lower your head, which will rotate your body to a horizontal position and allow you to swim underwater.
The other type of dive is the head-first dive, which can be started directly from a horizontal position while snorkeling. To be most effective, this type of dive should begin with a good amount of forward momentum. As you kick forward, bend at the hips and pull your knees and arms in towards your chest. Thrust your legs straight up and maintain a streamlined position to glide down into the water. Continue to kick with your feet to move down deeper, and simply arch your back to level off and or continue up to the surface.
Taking Care of Your Equipment
You can prolong the life of your snorkeling equipment and keep it in top shape by regularly soaking them in fresh water. Salt crystals can condense on equipment that has not been properly rinsed or soaked. These can dry and harden, causing scratches or holes in equipment and weakening straps. Check your equipment after each use for bits of sand or salt and rinsing all equipment thoroughly.To avoid breathing in water while you are using your snorkel, get into the practice of breathing in slowly and evenly, so that residual water in the snorkel does not enter your mouth. Exhale sharply and with force to rid the snorkel tube of water.
If Your Mask Fogs
Sometimes the inside surface of a snorkeling mask will begin to fog. This happens when moisture in your breath condenses on the cold glass surface of the mask. To avoid this, regularly clean both the inside and outside of your mask with soap and water to remove all dirt and grease. If your mask begins to fog during a dive you can clear it by allowing a little water to flow into the mask. Then look downward to wash the condensation from the lens and clear the water out of the mask.
If You Get Tired
If your legs become tired or if you develop a cramp while snorkeling try flipping over onto your back. This will let you tread water easily while remaining afloat on the surface of the water. The inverted leg motions will be much easier than the basic kick and will let your muscles rest and recuperate energy. Your body position should be semi-sitting, with the head above water.