Watery stools are a function of not having the colon anymore. Problems with leakage MAY be due to a sphincter issue, since it is much more difficult for the sphincter to hold-in watery stool vs. solids. If c diff and pouchitis have been ruled out, the next thing I would attempt would be to find a way to functionally make things work as best as possible.
Are you taking any meds to thicken your stool? I take 8 Metamucil capsules, 2 immodium, and a 30mg codeine sulfate about 3x/day, just prior to each meal. I DO have sphincter issues, and find this really helps thicken my stools.
This gives you something to try, if you haven't already-I would start with a lower dose and then work your way up until you find results or run into constipation problems
Another possibility may be that you do have sphincter difficulties-is your issue with urgency per say, or leakage and difficulty holding a full pouch in? (this can be hard to differentiate. I don't have urgency in that my ouch wants to empty so much as my sphincters cant hold it in anymore and once I start to lose control, I've got to go NOW.). Anyway, there are a variety of tests that can be done to measure the effectiveness of your sphincter muscles. A doctor can do a quick finger test to assess if this is something that should be further pursued with ultrasound, manometry, etc.