FWIW, I've been getting protected battery packs on eBay from
http://stores.ebay.ca/aokbattery/
The highest capacity right now are the Panasonic 3400maH.
You have to waterproof them yourself.
He can provide a standard connector or a waterproof screw-together one.
Many of the cheapo Chinese lights throw a lot of lumens but their battery packs are terrible, so folks are ordering the light heads and getting batteries from Hunk Lee at AOKBattery.