A U.S. Navy SEAL hopeful and his friend, an off-duty lifeguard, died while doing underwater drills that can lead to what is often called Shallow water blackout. The two were in a pool just 3.5 feet deep. No one realized anything was wrong until their limp, unconscious bodies were noticed beneath the surface. Shallow water blackout is also called hypoxic blackout. * Warning signs are appearing atpools prohibiting prolonged breath-holding. New York City and Santa Barbara, Calif., are among the first U.S. cities to outlaw long breath-holding in public pools.