way's why the Mercruiser and all the other I/O systems that use automotive
engines( that I know of) do have a circulating system, same as you car.,and
raw water flow isn't restricted by the thermostat on the inlet. They
actually have been engineered to account for your concerns and ,amazingly
they do work, with or with out "closed loop" exchanger system. The decision
weather to use closed loop fresh water system is more often a practical one,
if the engine is dificult (expensive)to remove or not is a big consideration
(plus freezing concerns). The point being, why spend something like
500-1000 bucks for the exchanger system to protect a cheap car engine? The
cooling system corrosion issues won't make the engine last forever anyway?
That is unless you engine is buried deep under some nice stuff rather than
being under a lift up engine cover as most of these setups are. To an extent
you could think of these ase thowaway engines. They are relatively
un-expensive and have limited useful life. Oh, but their cooling systems do
work!

--
Ron White
my boatbuilding web site is:
www.concentric.net/~knotreel