I bought 2 Israeli Tactical grips foom CDNN @ $14.99 a piece. Didn't know what I was going to get at that price but turned out to be a good buy. Has battery compartment & is very comfortable, ergonomic. http://www.cdnninvestments.com/arpigrnewism,html click on search (enter) AR15G
I have one. I don't know if I would trust it or any other grip to be "waterproof", but it works as well as just about any other grip. I prefer the Miad slightly with its bigger backstraps, since I have bigger hands.