The problems I've been aware of in our marina with break-ins have all be of a casual nature--- a cabin door or window was left unlocked or a lock was easily defeated.
Earlier generation boats of our make have a gap between the main cabin door and the door frame. The latch and the deadbolt are visible in this gap. On top of that, the deadbolt mechanism on the earlier boats like ours has a weak detent system--- it's pretty easy to simply push the deadbolt back if you can get to it. Even though we have long since replaced the original corrosion-prone plated door latch/lock mechanisms on both the main cabin and aft cabin doors with identical units made of 316 stainless, the inner workings of the deadbolt are unfortunately also identical.
Remembering back to my childhood in Hawaii where one of the houses we lived in had a similar setup on the back door that I could open with a pocket knife blade, I decided to try to defeat the potential for this on our boat.
The solution was very simple as illustrated in the photo. A friend cut out the piece of stainless for me and it is through-bolted to the door as well as glued to the door with 3M 5200.
The reason it does not overlap the door frame very much is that when the door is opened it misses the handrail by only a half inch or so. So the plate cannot stick out any further or it will hit the rail. But at least it covers the gap.
Yes, a determined thief could pry the plate back and perhaps break the wood enough to get at the latch and deadbolt with a knife blade or credit card. But I'm hoping that by making it a bit harder to get the door open, he'll just move on to the next boat.
When we stripped and repainted the door some years after the photo was taken we simply left the plate in place and painted it again.