Haha.. it must be something to do with current (like electro platting effect), these are aluminum materials, and those spots don't looks like corrosion, Any scientist here can help to enlighten us?
BTW, your torch looks like also a twist on and twist off design, I suspect for such design, the paint is an insulator, the body is the negative return path, when we tighten the front cap, the negative plate of the circuit (for my case of front twist on switch design) touches the top of the body and form a close circuit. When this torch is in the sea water, the water form a high resistant path shorting the body and the front cap, that is why the torch was lighted up dimly when in sea water, as well as such effect happened.
If I have a chance to get hold of 3 of this torches, I will try to do such experiment, will bring the 3 of them into sea water in these conditions:
1. first torch will not have battery inside.
2. Second torch will have battery and turn on all the time when in water
3. Third torch will have the battery but turn off all the time while in water like my poor torch in discussion here.
But since I don't have 3 of this torch, then no way to find out what is the real cause.