HI,
I think that to have a good physical condition is important for every sport activity.
The underwater activity for how much intense is, it's not sufficient to guarantee a good physical condition.
It's necessary to pratice a aerobic activity.
If you have a good physical condition, you can resolve more easy many of the underwater emergencies.
Moreover, a good physical condition, guarantees one inferior production of CO2, not negligible in our particolay activity.
Aldo