I traced the raw water line to the engine, all sorts of skinning knuckles pulling hoses in the bilge - blockage was in the engine. I took the line off the manifold. Oddly, the first gallon or two of water was white like skim milk. But not the kind of oily white of an oil/water emulsion. (I think). I ran the engine for 15 minutes, all good, but I did not think to check the oil
Still its had salt water running through it for 50 years, one of these days its going to rust through. Any comments gratefully received.