Doctor Who TARDIS Bath Mat
We may not see many of them, but there are actually quite a few bathrooms on the TARDIS. (According to the Wiki, 14 of them.) Considering how big the TARDIS is, though, you probably need a map to get to them. And, well, like the swimming pool, there's a possibility the Doctor may not be exactly sure where they ended up. Probably it's best if you start your journey to the restroom on the TARDIS c. two days before you actually need to use the facilities. Take a snack or stop by the vending machines near the console room on your way out.