What should a diver do immediately if they start to experience shallow water blackout?

When a diver begins to experience shallow water blackout, the most crucial action is to signal for help and try to stay calm. This reaction is important because shallow water blackout is a loss of consciousness caused by a lack of oxygen due to hyperventilation or prolonged breath-holding. By signaling for help, the diver can alert nearby companions to their situation; this is essential for ensuring assistance as soon as possible. Staying calm helps in managing the physical response to the situation, maintaining control over the body as much as possible.

Other actions like attempting to swim to the surface quickly or performing rapid ascents can lead to additional risks, such as barotrauma or further loss of consciousness. Descending deeper to increase air pressure would also be inappropriate, as it would worsen the condition by compounding the oxygen deprivation issue. Thus, signaling for help and remaining calm is the best response in this critical scenario.