Yep if the terminals are heavily corroded, that could be your problem, as far as diagnostics:
measure battery voltage, lets say is fully charged 12.6V:
1)now put your voltmeter + probe on the big terminal on the solenoid connected to the battery cable, other (-) probe on battery neg or engine ground...see what you got, should be close to battery volts. If not corroded terminals can be the cause.
2)next test: harder to access but the start wire is the yellow/red wire on the solenoid. If you can get your voltmeter + probe on this, ground the other (-) probe and have someone turn the key to start while you watch the meter. If its batt volts, could be starter or solenoid (which can be replaced separately), if its zero, the problem can be elsewhere in the starter system wiring. On some of these boats it can be hell to change a starter. Might have to remove the exhaust manifold on that side. If you have the old style heavy Delco style, consider replacing it with the lighter Permanent Magnet Gear Reduction style. Just make sure to use the right bolts and torque to spec. Loose or wrong starter bolts can break off in the block and cause you untold grief.
Suggestion:
if you have to remove the old one, once you have access, tie a rope round it to hold it up while you take the bolts out. They are heavy and the bolts are up under the engine hard to access sometimes.
Here's my old one, still running this old style Delco one but due to change it to a PMGR this spring.
https://www.dropbox.com/s/97y7b927tdgll ... r.jpg?dl=0I have used this one since 2007 and replaced the solenoid once because the exhaust manifold drain leaked water on the terminals (have since replaced these OMC one piece units with the style used on later OMCs and Volvos, no drain there to leak, its on the back end of the manifold). It's starting to crank slow so its time. I have an ARCO PMGR waiting to be installed as soon as it warms up.
After installing coat terminals with a good marine grease, I use OMC/Evinrude triple guard.