I would of course start with what has been suggested, but it can be a few other things I can think of off the top of my head... It can be the starter, starter selenoid, corroded cable, and the elusive electrical Grimlin that no one can ever seem to find..
The grimlin got me on one of my past boats... I had the same exact problem you are describing. nothing had any sign of corrosion.. starter and selenoid tested fine.. I never figured out the problem... My mechanic said I had a grimlin in my wiring..
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