There are some tough tests in the military which require an underwater swim. Here is what Stew Smith, a Navy SEAL who passed the test, had to say about it:
Even at SEAL training where the test is 50m without a kick off the first wall plus a forward flip before you start, NO ONE in my BUDS class failed that test. At that time we had 120 people in the class who took and passed that underwater test.
We only graduated 28 of them. There are bigger issues to train for like running, swimming, pull-ups, and many more that will get you versus underwater swimming.
Smith had some additional advice to give on the subject:
One last thing, NEVER hyperventilate prior to underwater swimming. In fact, the Navy will not allow it for the test — so do not practice it.
One big inhale / exhale and one last inhale are all you get prior to swimming underwater. The Navy will teach you this. No need to practice it prior to training.
In the big picture of your training, underwater swimming should be something you focus on minimally by learning superior underwater swimming techniques.
BUT do not push your personal envelope to pre-pass this test on your own without trained medical professionals on the pool deck with you.