Measuring the Voltage right at the starter motor (positive stud & motor case) will also tell you if you have any ground issues.
Measured the voltage, as you suggested, at pos stud on starter and motor case while turning key and got battery voltage (13.1V). Also, swapped to another battery (1000 MCA deep cycle I bought new in March 2017 for trollling) and got 13.8V. This was to rule out bad batteries.
Jumped the large terminals on solenoid with battery cables and nothing happened, key off and on.
Wire brushed and sand papered the lug on the block ground wire and block (also took stud out and cleaned and re-tightened), the 3 mounting brackets on solenoid and block, the battery end lug of negative cable, and the both battery terminals/studs. Also, cleaned the 2 small wires on solenoid.
Ran battery cable from negative battery post to starter bracket to try and establish a ground, nothing.
I'm still only getting a solenoid click when turning key. If I connect battery cables directly to starter positive stud and starter bracket it won't spin with starter mounted to block. But, starter spins great on bench using exactly the same method? Was told today by a local outboard mechanic that if starter works on bench with direct power, that starter is good?