Scuba concave convex

I think you have that backwards. My SCBA is convex, I have never seen one that is concave. I think if the bottom was concave they would stand up on their own and that might be better...

convex_concave.png

 
Look at a champagne bottle, bottom caves in to better withstand internal pressure.

Some SCUBA 200 and 230 bar bottles comes with a flat bottom which nessitates more material not to deform under pressure and in water the added weight is not a problem. But we landcrabs who has to haul our air around hates added weight.

So a pressure wessel with (outwards) domed ends gives the most strength for any given material = light as possible.