My guess is, you've reached a level of toxins that have been stored (deeper)in fat, (or at a deeper cellular level)and the body is trying to slow/ease their release, so as to not overwlem the systems of elimination--hence it is retaining water. Brilliant body!
I would probably drink more water, (which mightsound counter-intutitive,)but just sipping, not chugging. Might try sipping warm, boiled water. (which is my favorite on a wfast--like pure tea.)
And if I was feeling depleted and very weak,(doesn't sound like you are) I might take a bath with himalayan salt--IF i was concerned about mineral stores going very low.
On the whole though, I feel it's best not to 'micro-manage' the water fast--I never weigh myself, for exmaple. The body knows what it's doing. Weight can fluctuate--and might stay up for a few days. . . enjoy the process; it makes it that much richer.