Water isn't a necessity, but I would start giving it with food. Just to get used to using a cup, and drinking something other than breastmilk. I would expect it to be played with and not particularly drunk from for a fair while.
I've been giving DS a cup of water with meals to get him used to drinking water. He's also taken to food with gusto and is eating a lot. That combined with the recent hot weather gave him a touch of constipation. Extra water seemed a good plan for this too.