Hi walker_1912, I've camped up there quite a few times over the years but only in the winter and early spring time. I have had the company of other tents once or twice but in the main have had the place to myself (plus Woody the dog).
Water can be gotten from several run offs coming off of Great End this time of year. It's then up to you how you filter it, if you do at all.
I have never taken water from the tarn itself to drink, and never would personally. It is a popular lunch / camping / stop point for many walkers and who knows what they get up to in and around it.