Your snorkel will fill with water when you dive below the surface, so you'll need to clear it once you surface by using one of two methods.

The blast clear is the most popular method. This involves blowing out through the snorkel to force the water out through the top. If your snorkel has a purge valve, water will also exit through the purge valve below the mouthpiece.

Another method, called the displacement clear, can be used with snorkels not equipped with a purge valve. The displacement clear begins while you are still underwater. As you begin to surface, look straight up and exhale a small amount of air into the snorkel. As you ascend, the air will expand and displace the water. By the time you reach the surface, most if not all of the water will be cleared from the snorkel.