Lead sheeting is probably the easiest to use as it can be formed easily to the shape of the pod, but a good epoxy paint will still be needed between the pod bottom and lead sheeting because the air inside the pod will get saturated with acid fumes, and condensation will form between the sheeting and pod bottom.
Sealed batteries will reduce the effects of acid fumes.
On my boat, the batteries ( 14 ) are inside the cabin so I have them contained in stainless steel boxes with hinged lids and rubber gaskets. The cables and exhaust ducts have rubber grommets where they exit the boxes. It's not "waterproof" but is sealed up pretty good.
If I had "battery pods" externally I think I would put the batteries in stainless trays with rollers so the battery rack could be slid out of the tubular pods. Any spillage would be caught by the trays. The batteries must be secured to the trays, and the tray's roller mechanism strong enough so if the sub gets inverted ( OUCH ! ) the batteries can't move around.
I'm always concerned with being able to access all areas of the sub for re-painting so I don't make things "closed" where I can't reach it with a small grinder. I expect the saltwater environment will quickly attack all the surfaces both inside and outside the sub. Cleaning, grinding, and re-painting is part of an on-going maintenance program. There's lots of rusted "hulks" sitting around down at the harbor but with proper design and diligent inspection and repair a steel boat should last a long time.
Frank D.