I think that I first picked up about the water reflex from a related thread or two here on DV somewhere?
Basically someone was experimenting with breathing or going underwater, etc. Taking cold baths was mentioned also I think.
I noticed the thread last year which was about the use of water to help gain lucidity - except I've done a quick search and I cannot find it!

I did try splashing cold water on my forehead (for over 20 sec) during WBTB last year where I noticed that it became a 'dreamsign' which was incubated in a couple of dreams I had at the time!