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

Sometimes: There is no reason not to try this. However, usually cystitis ends up requiring antibiotics.

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.