1995 50HP 4 Stroke Mariner has an intermittent starter issue

[cpo1]

1995 50 HP 4 Stroke Mariner has an intermittent starter issue. It seems that the starter does not always disengage. At least that is what we think it happening. The first time it happened, boat was towed in and marina service company said the starter was fried. A new starter was installed and off we went again. Same thing happened again. After turning the motor off, there was a constant hum in the motor which we assumed was the starter again. We disconnected the battery and after reconnecting it, the boat started.But while the motor is humming (with the motor off - battery still connected) the ignition switch is powerless. After reconnecting the battery, the ignition switch worked again. This time, we were told it was likely a bad ignition switch. The switch was replaced and off we went again. Same thing happened again. But this time, we could not get the motor in forward gear. After disconnecting the battery a couple of times, we were able to get the motor started, in gear, and it now sits at the dock, awaiting our further options. Any suggestions will be appreciated. Thanks.