First, make sure you have it in neutral. Take it out and put it back in. Jiggle it a little.
Then you have to see if you are getting voltage to the solenoid trigger wire on the starter. It is the small wire on the starter. Turn the key to start and see if it shows any voltage. Unhook it and try again. Check the back of the ignition switch. Do you see voltage on the start tab?
If you don't have 12v on the starter trigger wire at the starter, trace the wire. I don't know about your boat, but on mine the starter trigger wire is yellow with a red stripe. The most likely trouble spots are the neutral safety switch and the ignition switch.
'96 ProStar 205 SD LT-1