What should divers always carry as a precaution during their dives?

Divers should always carry a surface signaling device or whistle as a precaution during their dives because these tools enhance safety and increase the likelihood of being noticed in case of emergencies. Such devices can attract attention from boats or other divers, making it easier for a distressed diver to signal for help.

A whistle is particularly effective because it can be heard over the noise of the water or wind, and is lightweight and easy to carry. Surface signaling devices, like safety sausages or reflective mirrors, can also serve this purpose, but having a whistle is a straightforward and reliable option for immediate distress signaling.

Other options, while useful in certain situations, do not provide the same level of immediate and effective communication for assistance in emergencies. A reflective surface may not be as easily deployable as a whistle, a large weight complicates response in emergencies rather than helping, and a map of the underwater area is not particularly beneficial if a diver requires urgent assistance.