I have been using 5mm. marine ply for my Thames barge keels.
I guess it depends on the size of the craft you are building . Mine are forty two inches and I find the five mil works for me.
Waterproof PVA glue goes well with it but I find CA does not hold well to plywood. Epoxy works too.
The two tools I find really useful are a dremel scroll saw
and my ryobi drum and disc sander combo.
They save hours of hand work.