If a diver's one and only computer fails underwater, the diver should:

In the event of a dive computer failure underwater, the safest course of action is to terminate the dive and ascend at a controlled rate, no faster than 30 feet per minute. This choice is based on the need to prioritize the diver's safety and avoid potentially dangerous situations associated with rapid ascent, such as decompression sickness.

When a dive computer fails, it eliminates crucial information regarding depth, time, and no-decompression limits, which are essential for safely managing the dive and planning the ascent. Without this data, the diver cannot accurately assess their current situation or the time they have remaining for a safe ascent. Therefore, terminating the dive allows for a more controlled and safer response to the unexpected situation.

Ascending at a rate of no faster than 30 feet per minute is also crucial, as this helps to minimize the risk of decompression illness by allowing time for inert gases to safely exit the body as the pressure decreases.

Other options suggest continuing the dive or making uncontrolled ascents, which could result in serious health risks. Thus, terminating the dive and ensuring a methodical ascent is the most responsible action a diver can take in the event of a dive computer failure.