Dry Top Snorkel Technology
U.S. Divers offers a variety of patented 'Dry' snorkel top designs that all close to seal out water when you go under water.
While snorkeling on the surface of the water just breathe normally. When you want to dive under the water take a deep breath and then submerge. The snorkel will automatically close and seal out the water from the tube. When you resurface just exhale and breathe normally. As long as you keep a good seal with your mouth on the mouthpiece your snorkel will not take in any water.