Hi

Tap water, unless your domestic supply is obviously too heavily chlorinated.

Water temperature should be sufficient to maintain your overnight ferment in excess of 23*C, but less than 27*C.   Only you know the variables particular to your environment [ie room temperature and flour temperature]

Best of luck

Andy

ps. overall hydration for ciabatta in this recipe is low.  But, see how you get on

Our tap water is highly alkaline and has a strong taste unless filtered, so I use filtered water. 

At my mother-in-law's or parent's house, where the tap water is sweet, I use it straight and don't filter it first. 

When I made this recipe about a month ago, I just used room-temperature everything. Turned out GREAT, and it was easy. Made me feel like a pro!

Good luck!