Such a simple drill, but carrying a good amount of teaching. Underwaters isn't just about finding out how long you can hold your breath;in fact, these can be quite dangerous if done alone, or to a point beyond discomfort. So, BE CAREFUL.
Underwaters is really about learning how your body works in the breaststroke underwater pull at the end of a race.
Be patient, and pay attention. Remember, this isn't about how far you can go without breathing, it's about how efficient your pullouts are so you can make it to the other end, or farther, without using any energy... energy that gets wasted in those momentum-destroying actions during the recovery of the arms and legs.