Using PVC or ABC for a seawater manifold is generally a bad idea. PVC is brittle and as it ages it becomes even more brittle. ABS is similar property wise to PVC except gasoline, acetone, etc will dissolve it over time (a very short time). So not good...
CPVC/ CORZAN pipe is somewhat stronger and less brittle, but I generally I would not use it below the waterline except maybe in a aluminum hull
Pex (cross linked polyethylene) might be OK to use below the water
line. Its stronger, flexible, Heat resistant. Not sure I like the crimp connections, etc, but the material is strong enough for below water
use. Good as marine
hose anyway.. PEX is tubing so 1/2" pex will fit inside 1/2" PVC. Pipe is sized by the interior
dimension, tube by the exterior dimension.
(not brass) is the way to go. Brass is copper- zinc based and the zinc can and will leach into the water over time. Happens even in domestic building plumbing
. Just faster in seawater. Bronze is Copper -tin based and is reliable but $$$$.