Locking devices gets damaged by salt water therefore its a good idea to put your locks in a place not exposed to the elements during a passage. During the years I have used several methods depending on the hatch. My hatches nowadays are sliding but they also can be clamped down into a gasket. Therefore they have to move not only forward but also a bit up and down. That complicates things a bit. To overcome that I use two padlocks, one on each side of the hatch. Of course it is very easy to break into a boat, even a small crewbar easily opens any hatch. The idea is to make the boat look locked. I think two padlock gives a better messages than one, but it is because Exlex hatch slides on ropes that I use two padlocks.

Pictures below.

One padlock in each end keep the hatch locked and hopefully deters undesirable characters.
A close up at one of the padlocks. These Finnish padlock has a robust mechanism. The string is to prevent the padlock from falling overbard when I am clumsy.
The two holes with nuts is to screw down the hardware in port when needed. At sea they are stowed below.

To be continued…

Regards Yrvind