[…]

Aha, at least that “plastic” should then keep out water from between lens and head, so something else might’ve been leaking.

Btw, trying a light underwater like that to test its IPXness is kinda like testing your blankets to see if they’re actually fire-retardant. If they’re not, well, you didn’t really need that house, now did ya?

If it’s a light I can fully take apart, I’ll do so and redo everything “properly”, including cleaning and relubing all threads, O-rings, etc., tightening all retaining-rings, and so on. I don’t take anything on faith.

If I can’t fully take apart a light (glued shut, etc.), I won’t assume anything about it, except to assume it is missing an O-ring or two (eg, the VG10Ses I got), and will leak like a sieve if dunked like a suspected witch.