PVC is no worse than other thermoplastics when it comes to thermal expansion, and is consideably better than some. I understand the concerns with internal finish, which varies greatly, but is chiefly to do with the way the pipe is manufactured and specified, and you get these issues with all types of plastic pipe, not just PVC.
My concerns with using aluminium in any wet area of a submarine is that it corrodes very badly. You could anodise the surface I suppose, that would offer some protection and also gives the opportunity to die the metal in an attractive or garish-depending on your sense of taste- finish. Seamless austenitic grade stainless steel tubing would probably be the best metal, but where do you find that in small quantities at reasonable cost? I expect they use it in fullsize subs, or perhaps the aviation industry.
I've found some types of plastic tubing that have an accurate and consistent internal tolerance ideal for piston tanks. The type used in silicone/mastic caulking tubes is very good, the PVC tubing used for condensing boiler flues is also excellent and also the tubing used to make stirrup pumps. Caulking tubes can be found easily enough for nothing, as builders discard them once the contents are used, but you are limited to a 46mm bore, which is fine for smaller models, but will restrict you if you need a big tank. The flue pipe is also very accurate, with a slightly larger 55mm internal bore, unfortunately quite expensive. I got my section out of a skip, but looked it up online- about £30 a metre! The stirrup pumps provide a much larger cylinder, but that's also a rather expensive way of purchasing tubing!
Unlike plastic plumbing pipe, these pipes have a high level specification for the internal diameter. Plumbing pipe is chiefly designed to seal around the external circumference, so that is where the high level of finish is. Has anyone had a crack at sealing around the outside of the pipe with simple commercial fittings? Wouldn't look as slick, and you will end up with a slightly larger diameter than internal sealing caps, but I can't think of a reason why it wouldn't work, and it could be useful for those without lathes.