If you look at the picture that is a couple of posts up you will see what I'm talking about. The two big red wires are on either side of the solenoid. One comes from the battery, one goes into the starter. The small black wire comes from the key and energizes the solenoid which causes it to slide up which does two things 1) it engages the drive gear in the flywheel BEFORE it starts spinning and 2) it completes the connection between the two big red wires allowing the power through to the motor so it starts spinning. If you dismantle the solenoid you will find the L shaped copper washers in there right inside where the two big red wires attach. Replace those washers and you will be good to go. You MAY even be able to adjust them as sometimes they are attached with an elongated hole allowing you to move them up or down. It's very likely that you will find one is much thinner than the other and you may be able to move it down so it makes proper contact.
Many people don't know about this simple repair and waste a ton of money on a new starter. I hate buying new things when a simple fix makes the old one work.
1990 Prostar 190, 351 Indmar SO, bought as an empty shell and rebuilt over the winter (04/05). Sold in May 2009
New boat - 1993 Prostar 205. Bought in better shape than the 190 was but still needed some work.