While marine theft is undoubtedly an issue, I don't believe there's much of a market for obviously-stolen MFDs, such as those with no mounts, cables, or documentation. So, common slash-and-grab petty thieves have few places to pedal their stolen wares; hence little desire to take them. And while they're typically dumber'n a sack of anvils, most are astute enough to overlook anything that has only a specialty market. Therefore, simple RAM mounts, or OEM MFD mounts for MFDs are usually sufficient. Easily-pilfered outboard motors, downriggers, crab pot pullers, etc. might be a different story.
I believe the old adage to "make them break glass" is the 95% solution to petty theft, particularly on a boat. i.e.-lock your pilothouse doors, through-bolt your outboards, and put anything of value out of obvious sight. Personally, I don't worry about MFDs on my flying bridge. I cover them for weather protection and little else.
And yeah-insurance is little solace for petty theft.