If you have the headlight out, finding the leak should be fairly easy. The headlight is made up of the front clear plastic and back black plastic. These seal together with a mastic like glue. The rear has one twist and remove cover for the indicator, one three screws to fit on cover for the Xenon bulb and one large black twist to remove cover for the main beam bulb. If you have these all in place, and the electrical plug sealed off, the unit should be water tight.
Inspect for any cracks on the front or rear plastics and check the rubber seals on all 3 rear covers are actually there.
Then, if there aren't any obvious damage routes, think about how water gets in. Water hits the front of the light and is then forced over the top, bottom and sides by air pressure. The volume of water you are talking about is a lot.
Has anyone gone at it with a jet wash?