I always lock myself inside, out of habit more than paranoia. Sneakaboards are not at all unusual in the Caribbean
islands, and I know of one boat in Turkey
that was backed to the quay overnight with the companionway
opened: no wallet, computer, camera
the next morning. This in a place where pickpockets and such are pretty unusual.
Personally I was boarded at 3AM at anchor
in Trinidad. I awoke to the faint sound of the hatch
stopping at the deadbolt. It turns out they were a couple of young teenagers primarily after the dinghy
gas but happy to take advantage of an open hatch
if available. I was VERY glad that I had developed the habit of locking the boat - it allowed me to call for help without having to confront them inside the boat in my birthday suit...
I mounted a quality dead bolt, such as used commercially (home quality would also do just fine) in the top of the drop board facing upwards: the bolt engaged a lip on the hatch, preventing it moving forward. It is very effective, strong, and difficult to circumvent.
Using a hasp and padlock to secure a boat is a bad joke: a large screwdriver can snap it open in a second with little sound. And anyone with an interest knows that, just as they know that people are in their deepest sleep at 3AM if they are after a wallet.