So I developed this complicated showering ritual that hinged on this waterproof cast cover, a shower stool, some towels, and a lot of careful hopping.  At first my mom could help, but I got quite good at it once she left.
I have a stand-up shower, so I'd gently hop myself into the shower, sit on the stool, and have my bum foot (in the cast cover) outside the shower on a folding chair.  It was a bit more like being waterboarded than showering, but it got the job done.  Eventually once I could do a tiny bit of weight bearing, I could bring my foot in with me, but wasn't worth the chance that I'd accidentally put weight on it.