Bolt it. Unless you swap motors often (more than once a year?). I used to live in an area that was known for crime/theft. More outboards were stolen off of the back of boats than any other item. Guys would show up in the middle of a given night and nab the motors, but leave trolling motors, fishing poles, depth finders, etc. Of the outboards that were stolen, not a single one of them were bolted. The ones that were bolted down are, to my knowledge, still on the boat.
I think there were 10 or 11 motors stolen in about a 2 month period. The pawn shops were full. That brings an interesting subject. Neighbor guy had an old Johnson 15hp outboard stolen off of his boat adn found it in a local pawn shop. He called the police, showed the cops his boat registration (with serial numbers for the motor) and the cops said ok. And left. The pawn shop owner had to show who they purchased it from, which might have been a fake ID or whatever. Ended up the guy had to BUY his own motor back. That's just how it works here.
The theft of outboards was so rampant at one point that people were sleeping under their trucks with rifles and shotguns waiting for the robbers to show up. And they did. After I moved out of that neighborhood, I saw in the paper where one of the robbers was hit in the knee with a piece of rebar from one of the homeowners in the middle of the night. Cops questioned him, had reason to believe he was involved and searched...and found enough evidence to put him away for a while. Probably 6 months at best knowing how the law works and how the lawyers will screw around.