Eh, a m³ of water doesn't even cost a Euro - not sure what it's in Japan, but let's assume a similar range. So that's what, max. 10€? Plus probably actually easier to clean than sheets and beds and such, and while the water needs more frequent replacement the pool itself probably doesn't need nearly as much maintenance ...
... I mean, I'd find it sorta unrealistic because such a pool would be very, very heavy and thus expensive to integrate into a building. But if you got that covered I don't think it'd be too much of a hassle. Possibly the biggest hurdle just the time to empty it and fill it up again.