Appreciate the help. the thought behind buying the scan tool was to split with a few fellow boaters. I was able to get some manuals for this engine and the merc manual for the diagnostic scan which say to use this tool from that seller. biggest problem im having is getting someone, anyone, to return a call. will price out the test as advised... if its even half price of tool I will probably buy it. have always preferred to do work myself and own the tools... have changed oil pumps, water pumps, trannys and so on in cars.. all i have been reading seems to point to seawater pressure but I would have thought that would have given me an increased temp... also have been reading that these sensors go bad. was told that scan tool can tell you if its a bad sensor but dont know for sure.
Thanks again. I will call the company in the morning for more info.