My experience is with Crohn's so I'm not sure if foods for UC would be any different. I was told years ago to stay away from soda, fruit juice, sugary foods and wheat. Definitely ask your dr for recommendations.
Staples like chicken, turkey and fish worked really well for me. They are also pretty easy to find on sale and freeze well.
Drinking water is great; especially to stay hydrated during a flare up. Check the label on the water you buy in the store. A lot of them are just tap water from local sources. Buy a filter like the type Brita makes. It might seem a little more expensive initially but you will save money in the long run and not have to lug home those heavy bottles! I got mine from Target; they go on sale all the time.