Hi!

When to opponents fire upon each other across a lake, they do not get +1 defense value. Is this also true when one or two of them are standing in the water?

Another related question: A stealther in water is stealthed alright?

Nope.

currently water is considered hindering only during movement not for range (which actually makes sense)

So no stealth in this case.

Stealth acts as hindering terrain for purposes of movement only. For the purposes of any LoF, the water is clear terrain.