I had a rain light failure because water had got inside the light and shorted it. It worked right up until I went out on the track and it got water inside it.
I use these, just stickytaped to the undertail - http://a.co/3QHwBAP or search Amazon for "LE USB CREE LED Super Bright Bike Rear Tail Light IPX4 Waterproof 5 Lighting Modes Red Safety Cycling Light - Fits on any Bicycles Helmet Backpack"
As you see above, the keyword "waterproof" is used, but there is nothing on it to suggest it is, ie no orings etc in it's assembly.
Anyone else recommend one which has been put through it's paces? I want something LED, and rechargeable via USB, not wired into the bike.