Underwaters are nothing more than gliding underneath the surface of the water. They are a common staple of many military commando units, such as US Navy SEALs or USAF Pararescue.
Subjects should focus on increasing lung capacity and practice increasing efficiency. Efficiency is going 25 meters in fewer strokes. Ideally,they should be able to go 25m on 5 or 6 strokes. Good stroke techniques to use are a modified breast stroke for the hands and either a frog kick or a dolphin kick with the legs. Sight should be focused at the bottom of the pool while staying alert to avoid crashing into the wall. Relaxation is important; it will enable conservation of oxygen. If the body is tense or flailing through the water, the muscles will consume oxygen that could be put to use increasing the length of the underwater. ALWAYS MAKE SURE A LIFEGUARD OR BUDDY IS PRESENT WHEN DOING ANY UNDERWATER DRILLS. Once the goal of 5 or 6 strokes in 25m is achieved, then try increasing the distance. 50m is the gold standard.