My Serfas USL-5 raider usb headlight failed after rain, should I attempt user service?
Don't attempt user service.  The rubber front-piece doesn't give access to the body, the case body is tightly shut, and there is no way in through the power button.  All you'll do is ruin the pre-existing water resistance by damaging the seal on the rubber front-piece.

Do leave the light indoors in a dry area and allow natural movement of moisture to clear the light.  In some cases this will allow the light to return to normal functioning after 2-3 days or more.

If you have removed the rubber front-piece, carefully and gently use a flat nose screwdriver or plastic probe to lift the plastic body of the light and try to use finger pressure to cram the rubber gasket back into place.  It will be held by plastic alligator style jaws that come together when the body of the light reseals.  Just be gentle and keep at it and you'll get most if not all of the way there eventually.