When you snorkel, you swim near the surface of the water while wearing a mask, fins, and a snorkel through which you can breathe. People like to snorkel in coral reefs, where they can see all kinds of ocean life.
If you've ever taken a beach vacation, you may have had a chance to snorkel, which is similar to scuba diving except that you stay very close to the water's surface. When you snorkel, you swim slowly, watching for interesting fish and other sea creatures, and breathe through a tube that's also called a snorkel. The word was originally a nautical term for a submarine air shaft, from German navy slang, Schnorchel, or "snout."