What with so many of us in the exclusive 'Cast Club', I thought I'd post about this.
I've recently taken delivery of a half-leg XeroSox dressing/cast protector.
Looks like this:

And basically, it's a giant condom that goes over your cast (also available for arms), you pump the air out, and it's waterproof enough to bath/shower/swim/suba-dive/snorkel!!
Click for Link. But, I bought mine
here, as it was cheaper!!
Not cheap - about £25 (not inc p&p). But, well made, and should last.
I've got 2 more weeks to go with mine. But, at least I can have a *proper* wash now!!
I'll gladly offer it out to others once I'm done.
Or, if you are already starting to 'hum', send yer other half round, and you can borrow it for the night!!