What action is essential during a diver's ascent to prevent decompression sickness?

Monitoring depth and time of ascent is crucial during a diver's ascent to prevent decompression sickness because it allows the diver to manage their ascent rate safely and effectively. When ascending from depth, the body is subjected to decreasing pressure, which can lead to nitrogen bubbles forming in the tissues if the ascent is too rapid. By keeping track of depth and time, divers can adhere to safe ascent rates, typically 9-18 meters per minute, and ensure they make necessary safety stops, which are vital for allowing the nitrogen absorbed in tissues to be released gradually and avoiding the risk of bubbles forming.

Other actions, like exhaling bubbles continuously, can be beneficial in certain situations, but they do not directly manage the ascent rate or depth consciousness as effectively as monitoring these parameters. Rapid ascent is hazardous and can increase the risk of decompression sickness, and holding one's breath can lead to lung over-expansion injuries, both of which highlight the importance of careful ascent management.