Hi TC. Because I've used a lot of it in my sub, I'd go with fiberglass
for a shallow ambient sub. It's real easy to do once you get the hang of
it. Pound for pound it's about the strongest, never gets rusty, is light
weight for a boat sized object, and is easy to patch or modify
later.
For attachments like thru-hulls it's very easy to bond metal fittings
into it. For a shallow sub you could even use schedule 80 PVC fittings for
many systems not requiring high pressure.
There are a slew of web sites that can give you strength yields on
different thicknesses of FRP but my favorite characteristic is the "never
rust" aspect.
It's the easiest material to form into sleek shapes too.
Take a look at my pictures on the psub site and there's a pic of the foam
mold I used for some fiberglass work. It was incredibly cheap to make the
mold, just required a lot of time hand sanding.
I'm actually surprised we haven't seen more shallow subs being built. The
cost of a dry ambient is probably 2/3 of a shallow 1 atmosphere sub capable of
say 60 to 100 feet max.
There are some things you need either way. Batteries, motors, scuba
tanks, gages, plumbing, and lots others. Depending on size, a 1 ATM sub with a
small pressure hull can be pretty economical.
I think the average K boat runs about $25K to build the basics.
Frank D.