Oh nooooo, you have to drink water and plenty of it. Our bodies are made up of water. Without it, our intestines would dry up. I'm 30 or so years of and living & dealing with IBSc and if I didn't drink my daily quota of water, I'd literally die..for IBSc I take Restoralax nightly.
If you have IBSd, your system is expelling alot of water from the diarrhea. So you need water. I'd look for a new dietician. One that truly knows about IBS c&d.
I've also learned that many Celiac sufferers also deal with IBS..eliminate high fibrous foods and perhaps get yourself tested for celiac disease(?). Good luck!

IBS-M is C + D together.

Sorry, I've only known about IBS c&d. Never heard of IBSm. Can you please elaborate?

I have been tested for Celiac and they say I don't have it. They did a 1/2 colonoscopy and told me I have IBS-M. They just told me to do the low fodmap diet.