The diver went below another 10 feet, but the guy joined him a minute later. The diver went below 15 more feet, a minute later, the same guy joined him.

This confused the diver, so he took out a waterproof chalkboard, and wrote, "How the heck are you able to stay under this deep without equipment?"

The guy took the board and chalk, erased what the diver had written, and wrote, "I'm drowning, you moron!"