Re: Merc 470
The 470 has a cast iron Ford 460 head bolted to an aluminum alloy block. Dissimilar metals, in the presence of fresh water (serving as an electrolyte), will cause galvanic corrosion. Change "fresh water" to "salt water" and the rate of corrosion will skyrocket. The aluminum engine block will quickly start to corrode; if its a later ('85-newer) style aluminum exhaust manifold, it will corrode; the aluminum water pump impeller will corrode; and due to the design of the water pump, the front nose of the camshaft will also be ruined. Once that happens, water will go past the front water pump seal and start dripping out the weep hole, and it will also go past the rear (oil) water pump seal and into the crankcase. It only gets exponentially worse from there.
In fresh water, he might would probably get away with it for a while, until the camshaft nose corrodes to the point that the water pump seals couldnt function. In salt water... the earlier mentioned 'days' estimate is likely pretty accurate. Unless you are out on the high seas off the coast of Somalia being chased by heavily-armed pirates, I wouldnt do it.
So what exactly is leaking? If its the heat exchanger, and decent radiator shop will be able to repair it, and if not, replacements pop up all the time on ebay or craigslist.
