When is it necessary to refer to altitude correction tables?

Altitude correction tables are necessary when diving at altitudes significantly above sea level, as changes in atmospheric pressure affect the way divers must account for their ascent and descent. At altitudes such as 900 feet, the atmospheric pressure is lower than at sea level, which alters the behavior of gases within a diver's body and the surrounding water.

Using these tables helps divers adjust for nitrogen absorption and decompression requirements that differ from those at sea level. This is crucial for preventing decompression sickness, commonly known as "the bends," which can occur if a diver ascends too quickly or doesn't account for the lower pressures at altitude properly.

In contrast, other scenarios listed, such as diving in saltwater, engaging in deep-sea dives, or using a diving bell, do not inherently require altitude correction because they typically occur at or near sea level. Deep-sea dives might require different decompression protocols due to the increased pressure at greater depths, but these don't involve altitude corrections like those required at 900 feet. Therefore, the requirement for altitude correction tables specifically applies when diving at altitudes where standard diving procedures need adjustment, making option B the most appropriate choice.