You need a diet that is non-irritating and easily digested - the point being to have very little residue left in your intestines before the MRI. That way they can get a better image. Usually it's a diet low in fat, fiber and lactose, which isn't too hard to do. As long as I can have peanut butter, I'm okay
The water enema helps them see the peritoneum better. It's similar to the procedure for an ileostomy reversal "leak test", but uses water, not barium. You lie on your side and they insert the tube into your rectum. There's a small balloon on the end of the tube to help hold the liquid inside. But don't worry about leaking. They put numerous pads under you before they even start the procedure and tell you not to worry about some leakage. It's not fun, but it's a necessary part of the exam, even with an ostomy.
MRI machines are loud so they usually give you headphones (with your choice of music) to wear while you're in the tube. There's also a way to signal the technologist if you're having a problem. It feels like you're in there forever so make sure you empty your bladder one last time before you start.
I would definitely tell them you have an ostomy if they give you barium or any other kind of oral contrast. See if they agree that you don't need to drink the entire amount since it isn't going past the ostomy area anyway. I hated when my bag filled up with a large volume of contrast that was meant for someone without an ileostomy.
Hope this helps!
Juliej