Given the way the "water" (the "leak") behaves, I think that it is the spacecraft.
More than that, I think it may very well be a TARDIS, one without its owner. (To which version of the Master -- we get to see two of them this series -- it belongs is not clear.) After all, it can travel through time and space. It is intelligent (we know, from a previous series, that the Doctor's TARDIS has a personality) and it can camouflage itself.
Note that the camouflaging has always involved the outside changing its shape, not simply changing its appearance; otherwise people would be bumping into bits of the TARDIS that were hidden, or inadvertently stepping through bits of it that weren't actually there. And this new use of the camouflaging has the benefit that it has (apparently) hidden its nature from the Doctor, so that instead of simply materialising in front of him in its final form, it appears as that "puddle" and then gradually changes to the shape of the starry-eyed girl.
As there has been speculation that the tear on Bill's face was some of the fluid, I was wondering if the tear is the Master's TARDIS in yet a new form. (If so, I'm half-expecting the line, "It's smaller on the outside!" to appear later in the series.)