What should each diver carry according to the final rule from Sheck Exley's Blueprint for Survival?

In Sheck Exley's Blueprint for Survival, the emphasis on each diver carrying three lights is grounded in the critical importance of visibility and navigation in cave diving. Having multiple lights ensures that if one light fails, divers have backups to maintain their visibility and safely navigate through challenging and often dark underwater environments. Adequate lighting is essential in preventing disorientation and potential hazards that can arise from insufficient illumination. Furthermore, by carrying extra lights, divers enhance their overall safety, as they can communicate with each other more effectively in low-light situations and reduce the risk of becoming trapped or lost in an unfamiliar cave system.

While extra air tanks, emergency flares, and diving knives are certainly valuable tools for divers, they do not carry the same immediate necessity for a cave diver navigating within constricted and poorly lit spaces. Extra tanks can be helpful for extended dives, but they do not assist with visibility. Flares may be used for surface signaling but are not applicable in an underwater cave setting. A diving knife can be important for safety, but its relevance is secondary to the need for reliable light sources. Thus, carrying three lights aligns best with the survival principle of ensuring ongoing visibility and safety in potentially dangerous environments.