You already know that your 12V and the ignition switch is good because it does try to start and sometimes will, so eliminate that. Since you already have to take the starter off for cleaning testing, replace it. it's unlikely you will find an alternator shop that will rebuild a marine starter and by the time you pay to havea marine shop rebuild it, if it is even possible, you've paid for a new starter.
Just replace it and go fishing.
2015 Seaark 1872 MV CC Etec-90
Two Helix 12 CHIRP SI's, , MinnKota Riptide ST80/i-pilot Link, Bob's Hydraulic
Bitter Gun Owner