A number of people around the world have reported problems with water leaking into the headpiece. The problem appears to be caused by the 3 screws that hold the headpiece together, which are too long for the holes they are screwed into, preventing proper "squeeze" on the o-ring seal. The cure is to remove the 3 screws and shorten them by roughly 1mm. The o-ring and sealing surfaces can be lubed before re-assembly. To prevent cracking the case, do not overtighten the screws. Cracks have been seen in headpieces that have never been disassembled, but so far none have caused leaks.
As a precaution, the cable should be sealed externally at both ends with urethane cement aka "Shoe Goo".
The repaired lamp can be tested by submerging in shallow water for 30 minutes, WITH NO BATTERIES INSTALLED, then drying the outside and opening it to check for leakage.
A lamp that already has water in the headpiece may exibit intermittent operation and shut itself off. The cure is to open it up, dry it completely until it works normally, then complete the repair described above.