The raw water cooled Mercruiser does have (or should have) a thermostat in
its thermostat housing located on the top and front of the motor. The
thermostat will be a 140 deg as the other poster said and you engine will
warm up and run fine. A "fresh water" system is helpful for keeping the
cooling passages in the block and heads from rusting thru or clogging with
chunks of rust but is not necessary to keep you engine at the proper temp. I
think one of the worst things about running such a set up in salt water is
not running the boat often. When these systems sit up for expended periods
big globs of rust can form and clog up something, like the exhaust elbow.

--
Ron White
Boat building web address is
www.concentric.net/~knotreel