One key difference is that humans need about as long as they were under water, at rest, to recover oxygen in their blood and normalize co2 levels. We can’t swim

One key difference is that humans need about as long as they were under water, at rest, to recover oxygen in their blood and normalize co2 levels. We can’t swim up, exhale, breathe, and swim back down. That’s a recipe for a guaranteed black out. We also can’t swim very strongly while holding our breath. Everything is slow, controlled, deliberate, and conservative in all ways possible.