Knock sensor
 

I just blew a motor melted piston
I did see the k knock sensor wire off
Would this retard my timing and cause damage

The knock sensor retards the timing when it senses detonation. If the knock sensor is disconnected the engine can be destroyed from an ignition system that is too far advanced (thus detonation).
Generally the knock sensor protects the engine from poor gas, a severely overloaded boat/overpropped boat or extremely hot weather.

My wire fell off I was looking around motor and saw
Wire off

Mefi?

Can you explain how an overloaded or over propped boat can cause detonation? How could one identify a boat operating in this condition? Mercruiser engines have a wide range of recommended operating WOT RPM depending on the engine.
Thanks.

A boat that is over propped will not reach the recommended WOT range. It causes the engine to work much harder. It like going up a small hill in an empty truck vs going up a steep hill towing 10k.
The engine works harder and cylinder temps increase which increase the risk of detonation.

Mefi #1