I FOUND IT!!!
Thanks for all the help guys. Building a dam with rocks was actually quite an effective method. I was able to get the water calm enough to see through, and using another flashlight I lit up the entire rockbed.
After being under almost a foot of water for a week I was surprised that it still worked. Lubricated o-ring seals held up fine, no water got inside.
FYI it's a Surefire E1e with a custom high CRI LED drop in module. Here's a map of where I found it. (the purple drop pin area, the blue dot was incorrect). The trail splits off a gravel vehicle road near the Lynn Canyon parking lot.