[vizakenjack]

When water is in a glass bottle, which is exposed to the sun, the water molecules also heat up by the bottle's walls.

But in waterfall, when water is in free fall, how does it get heated up? Does it absorb sunlight?
As far as I know, sunlight is composed of photons, right? Does it mean, that the outermost water molecules absorb all the photons, whereas the molecules "behind" get less energy from the sun (as such they heat up less)?
 

[Corribus]

Technically, but water absorbs so little visible radiation compared to what the sun puts out that this kind of inner filter effect is fairly inconsequential at these wavelengths unless the water is very deep. Water also absorbs IR and UV radiation emitted by the sun, and this is converted into heat. Another contributor to solar heating of (shallow pools of) water is absorption of surfaces underneath the water.

[vizakenjack]

Corribus, but how, on chemical level? I mean, this is chemistry forum...

So, in waterfall, or in ocean, the water molecules at the top... do they absorb photons?
Alright, so oxygen and two hydrogen molecules are hybridized this way. Which atom absorbs a photon?
If it's oxygen, does it mean its electrons in sp₂ orbitals ... get "excited" to higher orbitals...? Uhm, I'm confused how water molecules get heated up by sunlight.

[Borek]

It is not about electrons getting excited, more about vibrational energies of bonds.

Have you seen IR spectrum of water? It shows where the energy is absorbed. Every vibrational mode of the molecule that changes the dipole moment can get excited by IR radiation, no need to speak about electrons to explain that.