A series marine manifold.

Hi,
I think the water goes through the block first, then through the
manifold to cool it.  Then the water and exhaust exit via a riser
together.  At least that's how it works on my Mercruiser.  If at all
possible, use the radiator or a heat exchanger, especially in salt.
Marine exhaust manifolds(wet) and risers last 2-5 years, unless you use
$tainle$$ steel.   Good luck.  The A series so resembles a tractor
motor, I'm sure someone used it for marine apps.