6-8 BMs is actually a good number and pretty much the ideal norm for j-pouchers. Unless the BMs are always very watery, it doesn't seem like frequency is the issue for you. Keeping hydrated means more than just drinking water or keeping up your fluids. Electrolytes (primarily sodium and potassium) are needed to stave on dehydration, and an imbalance in either one can causes issues. In fact, you can actually cause problems by drinking TOO much water and flushing out the vital electrolytes from your system, so you may want to take this into consideration if you are pounding back the water nonstop. And remember, too, that other liquids (non-caffeinated) count toward your daily fluid intake, so you have to take the whole picture into consideration, not just the water you are drinking. In addition to keeping up your fluids, it's a good idea to include foods with that are high in both sodium and potassium. Sodium is in almost everything, especially canned and processed foods, so it's easy to come by. Good sources of potassium can be trickier if you're in a pinch (and I indeed had issues with low potassium when my UC was active and during my early days with the ileostomy, so I know all about that!). However, foods such as avocados, potatoes, tomatoes, watermelon, bananas, oranges, dairy products, fish and meat, yogurt, and blueberries are good sources of potassium, something to keep in mind.
Other guidelines: aim for no more than 3 caffeinated beverages a day. Caffeine is a diuretic, so caffeinated beverages don't count toward your fluid intake. Also avoid sugary drinks, and these include the popular sports drinks which may be high in electrolytes, but are also chalk full of sugar. If you are drinking juice, try "half and halfing it" with water, as this cuts down on the sugar. Finally, don't feel you have to pound back glass after glass of water. Sipping slowly throughout the day is just as effective. I usually keep a 600 mL (about 20 oz) bottle of water at my desk and sip it during the day. It helps me visually see how much I'm drinking. This is in addition to the vegetable cocktail and tea I drink during the day (I drink mostly herbal and decaf after my morning 1-2 "wake up" cups).
Finally, if you've already tried everything, you may actually want to get your sodium and potassium levels checked. This is more than just a single bloodtest, which tends to be just a snapshot of what is going on at that particular instant. You will actually have to have your levels measured over a period of a few days or weeks to really see if and how they are fluctuating (again, I had this issue with my potassium when I had UC. My levels could be normal and then suddenly drop very quickly, so my GI got a better idea what was happening when I had blood draws down over a few days).
Anyway, this reply got very long-winded, but I hope you get this sorted out!