I hadn't heard that a lack of water could cause diarrhea, but I have had it like yours, maybe once a day really nasty then I'm OK for a couple of days then back again - I try to drink lots of water but I slip on weekends - don't know if there's a link there - but I've been searching for a cause - it is bad and I've been caught far from home on a walk and barely made it back in time - not fun!