You should be drinking half your body weight in ounces every day (weight 150, 75 is half, then 75 ounces of water daily) for normal, healthy, daily living.
When deep cleansing with fiber, I increase that by 8-16 ounces with every dose of fiber.
The Psyllium & Bentonite act to deep-clean and pull off the layers of junk that are stuck to the lining of the colon (and neutralize the toxins), but P&B doesn't do anything to make your colon squeeze more. You will have more frequent bowel movements once the build-up is gone. So, it's VERY important to drink plenty of water when doing P&B shakes for a Bowel Cleanse (or ANY type of bowel cleanse).
A little constipation, bloating, gassy feelings and bowel "rumblings" are VERY common when starting P&B. You're definitely on the right track :)