Diver entered the water with tank valve turned off
At a nature preserve near an island, a group went out with four divers and two divemasters. The drop-in was a negative buoyancy back roll into the deep side of a vertical wall (800 msw, 2600 fsw deep). The divemasters stated that this was due to the currents. I was uncomfortable with this entry style with the experience of the divers in the group but said nothing. Also, the other divers were all in rental gear of generally poor quality. A loss of buoyancy on entry would mean we would never find the body much less be able to affect a rescue. As I was the only experienced diver in the group of clients, I dropped in first. Prior to entry, I checked all my gear and tested my regulator and inflator. I then dumped the air from my BCD and moved into position. The deck hand checked all my equipment and made sure my tank valve was opened. I rolled in and dropped down 6-10 feet (~3 m) and took my first breath. Nothing. Now I was underwater, slightly negatively buoyant, with no air for breathing or inflation. I kicked for all I had and was able to reach the boat and grab a hold of the swim deck. The deck hand was then able to reach over and turn my air back on.
Always turn the valve all the way on, or all the way off, and take a breath while looking at the gauge before entering the water. If the pressure gauge indicates you have gas, and it does not move when you breathe from the regulator, then you will not find yourself in the same situation as this diver.