The Japanese manufacturer Aqua Power Systems Japan is producing Water-activated battery in the AA and AAA sizes, with larger sizes and capacities on the way. As far as I can tell, this is the only commercial offering of these batteries in the AA and AAA form factors.
Liquid is injected into the batteries via a pipette, where it combines with magnesium & carbon to form the battery charge. It's rumored that these batteries have a 10-year shelf life while dry, and can be reused several times. They have a 500mAh capacity (Standard alkaline batteries produce 1700mAh or more), which is sufficient to power a flashlight for a while.
Sadly, many websites are getting distracted by the fact that these batteries can be posted by urine. Discussions of this product quickly degrade into a kindergarten mentality, and there aren't many serious reviews.
While the 500mAh capacity isn't great, I can definitely see this product for use in emergency kits. I keep a couple of flashlights in my car for emergencies or for late night hikes. Often these 'long life' alkaline batteries are often dead (or leaking) by the time I use the flashlight, which I only discover a year later while trying to repair a flat tire on the freeway at 10:00PM in the middle of nowhere. This seems to happen even if I leave the batteries in the original package. The temperature fluctuations inside a car probably worsen this problem.
This battery could solve the 'dead battery', because it remains inert and inactive until activated by water. And I have water in my car and in my home earthquake preparedness kits.
Pretty nifty technology.
I remember reading about a similar 'emergency' battery when I was younger. In this other version the battery remained inactive until you needed it. The battery contained two chambers separated by a seal. Water is contained in the top chamber, and the carbon & zinc cells in the bottom. You twisted the top, which broke the seal between the two chambers, and activated the battery. This battery didn't seem to go anywhere, and I have no idea if it ever reached production. I can't find it anywhere.