Either the starter solenoid contact has welded itself to the contact posts or the relay has stuck, if so equipped. Does your boat have a relay in the solenoid line (it should but not all boats do)? Either situation will give you a "stuck" starter. Usually after cutting power the stuck contact will drop out of engagement, but will stick again the next time you energize it. If it's a relay failure, its an easy replacement. If it's the starter sol, you CAN replace it, but you have to partially disassemble the starter to do so. Most people will replace the complete starter assy at this point. Hopefully it's a top mount.