The boat went to local dealer for diagnosis.
Turns out the speakers on either side of the cockpit were upgraded before his trip.
The larger speaker magnet interfered with the digital controls at the helm.{throttle and shifter controls]
The boat goes straight to guardian mode and can't be cleared. with the speaker in place.
It works fine with the speaker removed.
This was discovered by removing the speaker to access the controller.
I do not have all the details but that is what was discovered and Merc agreed with the conclusion.

Easy fix but only ****house luck led to the issue,

WOW, thats a weird one for sure ^^^^^maybe enclosing the speaker will solve the issue ?

Clarification from the boat owner...upon further inspection.

"The magnet from the speaker is pushing up against this connector. It's the shift position connector. I might only just need a spacer for the speaker.
Not magnetic interference."

But it was confirmed by Mercury it was an EMI issue...............

Beats me, I guess they deemed it a possibility,
easy fix anyway.