I think you've got some good advice for when you are on the boat. I rigged an even simpler companionway
gate using an upside down "v" of swaged heavy lifeline wire with eyes. Three lower eyes go over hooks under the companionway
corners and one in the middle. All three are then stretched tight and padlocked to a pad eye on the underside of the companionway slider. Anyone older than four can't squirm through. The key hangs just out of reach.
For off the boat, there are now nifty GSM alarms available on Ebay. You buy your own prepaid SIM (I use one from Tmobile in the US). A downside is if you are in other countries you would have to buy a local SIM.
The alarm will send you a text message or email
if the alarm goes off (you can hook up bilge
alarms, low battery
alarms, etc.). You can also get ones with a GPS
that will send you a text message if the boat moves more than a set number of feet. To arm and disarm you send your boat a text message. If you want to get fancy you could even send a text message to turn on a bright light to find it when you can't find the boat after a long night of partying. Buying
the alarm cost me about $200 and the monthly cost for the SIM is less than $5. No long term contracts.