Then just use the one u have without battery. If it get worst then u koe
You are right, next time when go diving will use the same torch and take a pic before and after dive for the 3 scenarios.
Interestingly there are many metal casing dive torch also using either head or tail cap twist on/off and they are ok. But from the close up look of my torch, it didn't seems to be sea water corrosion nor paint attacked by sea water. The paint (coating) on the torch looks more like platting process instead of paint, I suspect the current flow from the head through sea water then back to the body to form a closed loop, that process created a reverse electro platting process that dissolve the platting material into the sea water.
Therefore if there is no battery in side, there will not be current, then it will not get worse, secondly, if the torch is turned on, current flow directly from the body to the head and current will not flow through sea water which is higher resistance path. That is why I really interested to test it again to confirm my logic.