We used to have similar discussions when I was a scuba diver. I always dived with a full tank, despite the additional costs. Many other divers didn't.
My rational was that I wanted as much of the gas that would prevent me from dying on me for every dive.
And that was for a paltry depth of recreational diving of 130 feet.
But it was a good point about expense. Filling the tanks for the type of air replenishment system used in the submersible does have a significant cost. O2 ain't air.
