a) borrow the shower of a friend who lives three doors up and you know won't object.
b) get in your car, drive 10 minutes down the mountain, pay 400 yen and use the village onsen (read public bath with onsen water- nothing fancy).
Me I would definitely be the a option. And I probably wouldn't even call first so you could hide your dandruff shampoo and put down a new bathmat. Heck, I'd probably even forget a towel and borrow one of those, too. And I'd have to do it sneakily as I wouldn't realise I'd forgot the towel until after I finished my shower and was standing on your bathmat dripping. If I called out then you'd be sure not to hear me and then you'd stick your head tentatively around the door to see what I wanted and ehhhhhh!!!!
K? B. That's b with a capital B. K would absolutely under no circumstances borrow his neighbour's shower. And I was throwing all sorts of circumstances at him- the local onsen is closed, you have a big presentation on at work, the road to the big onsen has been taken out by a landslide and it's winter so you don't want to just use cold water. Even then???
Apparently even then. Wow.
Oh and just so you're not losing sleep worrying- our hot water service is fine, it's not cold outside and there are no local landslides (that I know of anyway).
We did however have a visitor this morning who left looking a little more polished and spiffy than when she arrived.