I have done both strenuous (Crossfit) and simple swimming, light walking during my water fasts.
I find I feel best by the simply walk or casual swim. Crossfit made me quite weak, and now that I have educated myself more concerning Water Fast I can see how it was not wise to Crossfit during the waterfast.
What I learn about waterfasting is when it cleans out the crud the body is not happy when one goes against it's instinct. I feel our instinct is to farm. The body likes movements that coincide with gardening as well as chasing kids. It isn't in my nature to run marathons or olympic style powerlifting.
I try to stick with this kind of movement while water fasting. Since it is winter and I am not gardening then I will enjoy the local pool to walk the lanes or do simply strokes. I find it helps the toxins go out of my skin and blood better than just laying in bed all day.