During an ascent, what is the critical action every diver must remember?

The critical action every diver must remember during an ascent is to slowly ascend to avoid decompression sickness. When a diver is underwater, the pressure increases, which causes nitrogen to dissolve into the body's tissues. If a diver ascends too quickly, the rapid decrease in pressure can result in nitrogen coming out of solution too quickly, forming bubbles in the bloodstream and tissues—a condition known as decompression sickness, or "the bends."

By ascending slowly and following the recommended ascent rates, divers allow for the safe off-gassing of nitrogen, which significantly reduces the risk of developing decompression sickness. This is typically guided by safety stops, usually at 5 meters (15 feet) for several minutes, allowing the diver's body to acclimatize and safely release nitrogen.

Swimming during an ascent can be counterproductive, as it may cause the diver to ascend too quickly or inadvertently stray away from the ascent path or safety stop. Paying attention to dive computers is important, but focusing solely on the computer without observing your surroundings or maintaining a controlled ascent can lead to dangerous situations, especially in case of emergencies.