One bit of preventative maintenance I did on my old Universal diesel was to have the starter motor rebuilt at about 1000 hours.
It was starting OK, but a mechanic friend advised me to fix it before it broke. (!)
Interesting thing was that when reinstalled, it turned Noticeably Faster ! Parts like that can have their performance degrade so gradually that you do not realize how bad they are because you have no recent frame of reference.
The shop that did the work replaced the solenoid too, since on my model they are kind of a one-shot device.
Not expensive by "boating standards"... at around $135.
Worst part of the job was the R&R. That upper bolt took an extension and a wobbler. And some mild cursing.
OTOH, if yacht maintenance was easy they'd just let anyone
Also, as noted by other posters here with band aids on their knuckles, DO remove all of the ground lugs and sand paper every one and reinstall tightly. 12 volts is not all that eager to push its way thru a tiny bit of corrosion or dirt..........