89 Mariah Starter Firing Randomly

[Digger002]

Out of nowhere this season my starter in my 89 Mariah (454) started engaging on its own once I insert my key and simply unlock the switch. Mind you I am not turning the key to engage the starter. The boat will actually start just fine but after it does the starter will attempt to re-engage and grind on the fly wheel. So being an older starter I ponied up and replaced it but the problem persists. Anyine have an incling as to what might be causing this problem?