Underwater Knot Tying is a commonly used training method found in some military courses, such as US Navy SEALs and USAF Pararescue.
The purpose of underwater knot tying is to train individuals to perform specific, detailed tasks while underwater. The idea is if they can tie rope into various configurations, they have successfully demonstrated the ability to maintain awareness, perform a detailed task, remain calm, and have developed the lung capacity to facilitate those things. In cases where no rope is available, you can substitute the activity with ditch-and-don drills, where the individuals ditch their gear at the bottom of the pool in a calm, purposeful fashion, or descend down to the bottom to retrieve and don their gear before resurfacing. Other things include configuring bricks in a certain pattern before resurfacing.