Can I cure cystitis by drinking lots of water so I pee more and flush it out?

Antibiotics better. Cystitis can sometimes be cured by drinking lots of water and urinating frequently, but may take a long time + could become chronic cystitis. Antibiotics are usually rapidly effective, often with a single dose, 3 day or 7 day courses of Nitrofurantoin (macodantin), trimethoprim-sulfa (bactrim or cipro (ciprofloxacin). All should be inexpensive and safe provided you aren't allergic.
Sometimes. There is no reason not to try this. However, usually cystitis ends up requiring antibiotics.