the answer is, it depends.  to the top of the structure

at 70 feet?  sure.  to the flight deck at 140 feet?  it'd be a

waste of time.

 let's do some air planning:

using my SAC (surface air consumption) rate of .8 cf per minute

at 1 ATA, this is what comes out:

descent gas: getting down to 140 feet (at 50 fpm): 9 cf.

rock bottom gas (enough air to get me and my buddy

to the surface should he have a total air failure):

2 minutes at 140 fsw to sort the problem: 16cf
1 minute to ascend to 110 fsw: 9 cf
1 minute to ascend to 80 fsw: 7 cf
1 minute to ascend to 50 fsw: 6 cf
1 minute to ascend to 30 feet: 4 cf
1 minute stop at 30 feet: 3 cf
1 minute ascent to 20 feet: 3 cf
1 minute stop at 20 feet: 3 cf
1 minute ascent to 10 feet: 3 cf
3 minute stop at 10 feet: 6 cf

total rock bottom gas: 60 cf.

total descent and reserve gas: 69 cf.

that leaves me 11 cf to actually use at 140 feet on an AL 80,

which for me is shy of a 3 minute dive (i.e. not gonna happen).

to dive to the max of my NDL with 26% O2 at 140 feet (10 minutes),

I'd need an additional 31 cf (for a total of 42 cf bottom gas).

so, to make a dive to 140 feet for 10 minutes, i'd need a minimum

of 111 cf. that suggests a steel 120 tank.

of course, that doesn't even address whether i'd want to go down

to 140 feet on nitrox to begin with (being a narcosis wus).

let's be safe out there!