Using Battery in Water

There's a seawater battery that uses the water as one electrode. One disadvantage to it is you can't stack cells in series. :>)

Batteries can't really be said to work underwater. The problem is that the water acts as a conductor and allows current to flow between the electrodes. This also causes oxidation/reduction reactions at the electrodes, which degrades battery performance. In saltwater the performance degradation is rapid and extensive. I have never seen what happens to batteries in typical tap-water. I expect that the effects would be much less severe.

AFAIK, it is not dangerous to put typical low-voltage batteries in the water. The batteries will not explode or anything. And they do still deliver some power to an attached load.

While I don't think putting batteries in the bathtub is dangerous to people who may be in the tub with them, I do believe that if you want to design a device to function from battery power while in the bathtub, you will have to put it in a water-tight housing.

--Mac

Yes, a battery in a watertight enclosure.