The batteries won't explode if you store them while discharged. It can ruin them, but that's another story.
They can explode when you recharge them.
When you pass electric current through water, it will break down the bond in some of the water molecules into their base atoms, which are hydrogen and oxygen.
The hydrogen is very explosive, and if there is a spark of any kind, it can, and probably will, ignite explosively.
Remember the Hindenberg.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.