How Do Water Molecules Change the Path of the Wave?

You can only really think about photons as particles when you're observing them. The photon has many possible paths between the source of the light and where you're detecting it. There's not a single story for what happened to that individual photon. If you average up over many, many photons, millions of photons, then you'll get these sort of average behavior from a classical wave. But for an individual particle, it's a little bit random.