1983 Bayliner 1950 CAPRI

You need a buddy to test voltage when the key is turned to "start" position. If voltage drops to below 12V, you either have a bad battery or a bad starter. Also, while holding it in the "start" position, have your buddy tap the side of the starter with a hammer. If that makes it crank, you have worn bushings and/or brushes in your starter.
make sure the neutral safety switch is closing as well....there are other things that could be contributing but you don't say what is in the drive train...