What is a potential hazard when not equalizing effectively during a dive?

When divers do not equalize effectively, they may experience ear pain due to the pressure differences that occur as they descend. As a diver goes deeper underwater, the surrounding water pressure increases. If a diver fails to equalize the pressure in their ears with the external water pressure, it can lead to discomfort and pain, often described as a feeling of fullness or pressure in the ears. This condition is often referred to as "ear barotrauma," which can be quite painful and, if left untreated, may cause more serious injuries to the ear structures.

While other choices may relate to diving safety, they do not directly associate with the immediate consequences of ineffective equalization. For instance, decreased oxygen levels and hypertension are not directly tied to the mechanics of equalization during descent, and loss of buoyancy typically involves issues related to equipment or gas management rather than equalization. Thus, ear pain is the primary hazard that arises from failing to equalize pressure properly during a dive.