I am informed creatine retains more water in your muscles, which made me think it could be bad for prostatis?

No: Excessive of creatinine use and muscle mass will create a chronic dehydrated state unless you drink plenty of water with it. It will also put an extra burden on your kidney. It has no effect on prostate gland but concentrated urine may mimics prostatitis symptoms. If you decide to take it, drink plenty of water (>3 liter/d), check kidney function and CPK periodically. Talk to PCP before using it

Creatine & prostate.: Creatine in doses of 1000 to 5000mg daily has been shown to increase muscle mass, muscle size and muscle energy stores. Creatine must be taken with lots of water throughout the day, and doses must not exceed 5000mg daily to avoid impairment of kidney function. The water is drawn into the muscles, but has not been shown to be harmful to the prostate.