on a 29 day wfast, I had BMs three times total. The third one on the last day seemed to be brought on by just thinking about eating, which got peristalsis going. The other two times were about five days in and midway.
Some people might want to do simple enemas. I never did. Autolysis will take care of a lot of waste that accumulates, which will be of the cellular debris sort, and will be burned up. So, it really comes down to the state of your colon when you undertake a wfast.
You would not use the SWF (Salt-Water-Flush) DURING a wfast, as the salt would be disruptive to the intricate balance the Water Fasting body enters into, and its managing of rexerves, etc. But I like the idea of (eating light before, and) doing a SWF (Salt-Water-Flush) on the morning of day one, say.
Or even, as in ayurveda, doing a SWF (Salt-Water-Flush) three days in a row, combined with light, or raw food, as preparation for the water fast. Then, let nature decide on whether to have BMs or not. I think it's pretty common to have one or two during a water fast. So much depends, always, on the state you're in, prior to the fast. Only you can know where you are at. . .
PS. I have never taken an enema -- the fasting masters seem to be divided on whether to do so or not. But remember, without food going in, a lot of the 'reason' to eliminate is removed. The body IS pretty amazing, and doesn't really need us micro-managing it--when it is allowed the space to water fast.