By definition:
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Snorkeling is the practice of swimming on or through a body of water while equipped with snorkeling equipment.
If we're talking from experience, snorkeling is a simple way to enjoy diving. You can see what's hiding under water with minimum training and equipment. All you need to know is how to swim and to hold your breath if you want to take a closer look at something that's a little bit deeper than you can reach from the surface.