I guess I’m confused by this problem, as treading water is a factor of fitness.
I used to sink like a brick to the bottom of the pool as a kid. In fact, I flunked Sr Life Saving Class three times only because I couldn’t float on my back without kicking or moving your arms - as required. I could however hold my breath until my feet landed firmly on the bottom of the 10 foot deep pool. "Arch your back" they’d tell me (which has zero impact when you’re that fit).
Regardless on being able to float, the test in today’s question is Treading Water. Your buoyancy will effect the energy that you have to put out, but that’s not a reason to fail this. So if you can’t tread water for 15 minutes (or however long the test is), then it’s time to start working out. In my opinion, any Dive Master (or Rescue Diver) should not only be able tread water but also be tested in swimming a given distance with their head high out of the water. You can’t help a person in need if you can’t find them so this is a vital skill.