Answer
Trying to use a long snorkel to breathe in the air from the water surface would not work. The diver would not be able to breathe the air into the lungs as the water pressure applies such a high pressure on the diver compared to the normal atmospheric pressure at the surface.
Work Step by Step
When a diver is underwater, there is a higher pressure than the normal atmospheric pressure at the water surface. For example, a depth of 10 meters underwater has about twice the pressure than the normal atmospheric pressure at the surface.
Divers use tanks to breathe underwater. The air in the tanks is at a higher pressure than the surrounding water pressure as this helps the diver breathe the air into the lungs. Trying to use a long snorkel to breathe in the air from the water surface would not work. The diver would not be able to breathe the air into the lungs as the water pressure applies such a high pressure on the diver compared to the normal atmospheric pressure at the surface. In fact, the famous oceanographer Jacques Cousteau actually tried this method with disastrous results.
It would be like trying to blow air into a balloon while someone is squeezing the balloon with both hands. We would need high pressure tanks to inflate the balloon in this situation.
