Was the electrician aware that the Santorin has a floating negative? Amel is one of the few brands that does not have the battery negative connected to the engine. I'm not on my boat now, but I think there is a green and yellow grounding wire coming of the top of the C Drive.
The through hulls should be grounded. Did he check the keel and/ or the copper strap going down into the bilge? This can rot through down at the bottom of the bilge and be unseen. It fastens onto a keel bolt in order to keep the keel grounded.
The Santorin is a great boat. Have fun!
Ian. Ocean Hobo SN96