A carry permit probably works best in most situations! If a thief wants that unit bad enough and has a little time, lock or no lock, they can get it. If you have to park your boat outside for any period of time unattended take the unit off. JMO!
I agree with Earl. If I am going to be away from my rig for a little while, I remove the units and lock them up in the truck. If someone sees it and really wants it, they will get it even if they have to break something to do it.
I'm with the popular vote on here. I can only speak for Humminbirds in that they're too easy to simply remove. One connection in the back and take it off of the gimble brackett.