Context: a man from the past gets reincarnated in the modern world, gets bewildered by a mirror.
kagami.... kono {{{mizu no} you ni} suki-totta} mono ga? 鏡・・・・この水のように透きとおった物が？ Mirror.... this thing [that] {is clear {like {water}}}? (literally.)
A mirror? This thing is a mirror?
Here, the character is comparing the properties of the mirror to the properties of the water, except the property is a verb, suki-tooru 透き通る, "to be transparent," "to be clear," in the sense of letting you see through it (hence why tooru 通る, "to go through," is part of the compound verb).
Considering the sentence, I guess the mirrors he knows about, from thousands of years ago, probably looked different, and weren't as clear as water the way the modern mirror is.
