An unseen hazard is a potential danger that is not easily visible or detectable. It can refer to a variety of things, such as a REEF or SNAG. A REEF is an underwater rock formation that can be hazardous to ships and other vessels, while a SNAG is an obstruction in the water, such as a submerged tree or branch, which can also be dangerous. Both of these unseen hazards can cause serious damage if not avoided.