What action should a diver take if they begin to experience symptoms of decompression sickness while still in the water?

When a diver experiences symptoms of decompression sickness (DCS), the most appropriate action to take is to switch to 100% oxygen if possible and follow the established treatment protocol. Administering 100% oxygen helps to reduce the size of nitrogen bubbles formed in the tissues and bloodstream, which are the primary concern during DCS. This is crucial because oxygen enhances the elimination of nitrogen from the body and helps alleviate symptoms while the diver is kept under controlled conditions.

In addition, following a treatment protocol after oxygen administration is critical because it ensures that the diver receives the appropriate care, which may include managing the symptoms more effectively and preparing for transport to a hyperbaric chamber for further treatment if necessary.

The other potential actions would not appropriately address the situation and could exacerbate the diver's condition. Descending further, for example, may provide temporary relief but can worsen the overall risk by increasing pressure and complicating the situation. Doing nothing and hoping symptoms resolve is misguided because DCS is a medical emergency and requires prompt action. Surfacing immediately without caution could lead to further complications, as the diver may not adequately address the nitrogen bubbles, increasing the risk of serious injury or even death.