I find this topic to be interesting.
I would like to see photos (or links) of recommended (better) locks and associated security devices such as the
grill insert that MarkJ uses.

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But...
Much of the previous discussion seems to be coming from several argument positions (which I am
posting with some paraphrased statements to illustrate the positions:
1. Don't Go There Policy
"I don't visit unsafe places."
"Just avoid the bad places."
2. Open Door Policy
"I never lock my doors." "I live where I don't have to lock the door."
"A lock will not stop anyone, so they are useless."
3. Minimizing Damage to My Boat Policy
"I would prefer they enter and take stuff without damaging my woodwork by busting the
companionway."
"It is only material stuff I can easily replace. I say let them take it."
4. Minimizing Risk to Myself or My Family Policy
"Security Comes from Preventing or Delaying Entry"
"Any obstacle that alerts me (by noise) or delays the entry of intruders helps me."
"I would rather have any amount of extra time to prepare a defense of myself, family, and boat."
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It seems to me that some of the arguments are mixing up things by changes of scenarios and what may be appropriate for one situation may seem overkill or inadequate in another situation.
For example, one person may be concerned with locking up an unoccupied (and presumed empty) boat in
storage. Another may be concerned about losing their irreplaceable possessions (while cruising or at anchor). Another considers the risk of what may happen (rape/murder/personal injury) if an intruder comes aboard while the owner (and family) is asleep.
From my perspective, those are entirely
different situations that could lead to different considerations of what is adequate or the best level of locking/security of the companionway.
Then too, the location of a boat, whether it is in a
remote (possibly risky) place or isolated in an anchorage, or in a crowded moorage or marina (with neighbors a few feet away) would seem to make a difference to me. I suppose that would be considered "situational awareness" to some. Does one feel safer in a marina in a crime ridden major city, or in an anchorage in a third world country?
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My personal position on this issue?
Short answer: A good security device is better than nothing. So, I fit more with Position #4 above, Minimizing Risk to Myself or My Family Policy.
I think locks of any kind simply indicate that the door is not open to entry to anyone (but the owner of the key). Locks (or similar devices) will deter some, but not all intruders. But,
having NO lock will only deter the honest people. Since honest people won't enter anyway, the only concern is for deterring (or possibly preventing) or delaying entry of the dishonest and possibly dangerous.
My position is reflective of the fact that while I grew up in a house where the doors were never locked, I have been burgled (twice), and had important things stolen from me. So, I like the idea of a simple "security grill" or bars that provides more resistance to entry, early warning of entry of intruders (due to noise), and yet also provides ability to allow for ventilation.
While a crowbar may defeat any lock or grill on a boat, IF I am aboard the boat (e.g. sleeping while at anchor), I would like to hear (be wakened and warned) while the intruder is trying to force the more difficult obstacle. If it gives me a few minutes of warning, that is valuable as it may give me the time to warn off the intruder in a non-lethal manner.
So, I am in favor of seeing more examples (hint: photos) of companionway security devices (grills, bar installations, locks) within this topic's discussion.