I'm not a medical professional, but I believe that in the case of an ileostomy, the reason you need to remain vigilant concerning hydration is because the colon is the organ which is mainly responsible for absorbing fluids. If you don't have a colon, your body is not as adept at attaining the fluids necessary to maintain homeostasis (balance) and good health. I can't speak for how much or how little your body needs fluids, but if you are *feeling* okay, and your doctor says it's alright, then I wouldnt' fuss over it too much.
I can speak from experience though and say that if you should ever get any kind of gastrointestinal bug, start drinking water, sucking on popsicles, ice cubes, soups - anything to keep your fluids up! My husband had his ileostomy for five months when our whole house became suddenly ill with what we later found out was a Norovirus. Every one of us got over it with little problem, except for my husband. He ended up in isolation in the hospital with what they were calling "acute renal failure" - and this after only one day of vomiting and violent diarrhea. I mean, when you're talking about kidney failure, this is not something to play around with....
He made a full and total recovery in a matter of days, once the virus had run it's course and he was IV hydrated. But still, don't mess with the prospect of dehydration. It's not pretty and sometimes doesn't take long for someone with an ostomy...