I put on about 30 lbs of "water weight" in about 4 weeks on my last round of high-dose prednisone, and I think most of it was on my belly. It was actually quite painful from the skin stretching. Diuretics helped a bit, but the only thing that really made a huge difference was getting down to about 20 mg a day. It's no fun at all, so I feel your pain.
Incidentally, at the time someone heard me belly-aching about it (no pun intended, I swear!) and very thoughtfully gave me a really great shea butter from L'Occitane, and I escaped that round without any new stretch marks.
My water retention didn't come and go though - it was just arrived, stayed for awhile, and then left (more or less.) Prednisone can cause accumulation of fat in specific areas though, ad I think the belly is one of them, but again, I don't think it would come and go - does your doctor know it's not a constant swelling?
Lisa