What is the first step if a diver on oxygen is underwater and indicates issues?

Switching the diver to 100% oxygen if available is important because it can help to address potential issues like decompression sickness or carbon dioxide buildup in the diver's system. Oxygen is a key treatment in these scenarios, as it can facilitate the elimination of inert gases that may have accumulated in the diver's body during the dive.

When a diver indicates issues while underwater, they may be experiencing symptoms that could escalate without proper intervention. Providing pure oxygen can aid in the rapid removal of excess nitrogen from the body, which is crucial if the diver is showing signs of decompression sickness, as it addresses the root of the problem rather than just managing symptoms.

In contrast to immediately bringing the diver to the surface, which can sometimes exacerbate certain conditions depending on the situation, the option to monitor for additional symptoms may delay necessary intervention. Ascending to shallow water could also introduce complications regarding gas absorption and pressure changes. Therefore, utilizing 100% oxygen serves as a proactive and effective step to stabilize the diver until further assistance can be rendered.