I have a Cateye Opticube that is waterproofed to 50m. I never biked that far below water, but the light was once dropped from waist high, breaking the cheap plastic holding the waterproof magnetic switch in place. So now I can only turn it on or off with a wave of a strong magnet; I have a small pile of now mostly useless bike lights.
That said, Bovine Oaks has convinced me to get even brighter lights. At cost, they are still cheaper that a tank of gas and fewer people die.
Bike lights that are supposedly built for "bike riding" are probably one of the most frustrating things about the industry, i personally think they are all crap or overkill.