If this boat engine (460) has an aluminum intake, I personally would start there, the first reason is that the aluminum intake manifolds, if not marine, have a tendency to corrode right at the front and rear water passage from the head to the manifold, reason 2 is the manifold is one of the easiest parts on a ford to remove and you can see the valley and inspect for cracks from freezing water in the block.
Before you do that, plug all the water inlets and outlets from the engine and apply a little bit of air pressure, no more than about 10 PSI, it should be able to hold it, you will know instantly if its leaking.
I'm betting on a leaky manifold to head gasket or possibly a cracked block (hope its the first).
Goodluck and keep us posted.....