This festival accomodate near 200-300 parcipiants - it's only active members, not audience. Moreover, this festival is described as well-designed performance with a good relations from town authorities and citizens.
Drink water is everywhere in shops from ~15rur for 1L, also You can drink free and take a water from a water columns - it is clear from artesian wells in almost every village. The filter isn't neccessary if You don't drink from Volga river or swamps.
The food is cheaper than in Moscow - launch costs about 200-300rur, and dinner 300-400 rur in cafe. I can recommend You a cheap self-service cafe in Uglich near city central. Will You sleep in tent or in hostel? Festival's providers usually make a big tent camping in 5km near the town. Take a lot of repellent!