Why is blue the last colour to disappear underwater? I have been told for years about how red is the first colour to "disappear", or apparently become unobservable, when diving to a significant depth beneath water.
When I recently did some diving myself, I noticed this vividly, seeing the blood of my nails and hand become apparently blue... Why does this occur? (I understand that this is a scattering effect but would appreciate a more in-depth explanation, e.g. whether this could happen in air...)
On a similar note, my father recently pointed out to me that during sunsets, blue flowers become seemingly more vivid in their colours - I assume that this is a similar scattering effect (related to Rayleigh scattering?), but what is it?
 A: The reason is that red light is more readily absorbed by water than blue light (by orders of magnitude). The physical reason for the absorption is molecular transitions - in particular overtones of vibrational bands. It has nothing to do with Rayleigh scattering, which is more effective at shorter wavelengths, but is quite negligible here.
Since there are fewer red photons when you are below a depth of water, then everything will appear bluer than it does above the water.
Some more details about the water absorption spectrum can be found here.
A: Red light is absorbed and scattered more readily than blue light in water (and in air too). As you descend fewer and fewer red photons are present since they have been scattered and/or absorbed above you. 
Typically this can be modelled with the Rayleigh scattering law for small scatterers that suggests a light attenuation as $I_0 e^{-k z/\lambda^4}$ - shorter wavelengths are much more strongly scattered, where they will add to the ambient light. (This, together with selective absorption, is also why the ocean is blue, rather than reflected sky - the blue colour comes from multiply scattered light that returns to the surface) 
OK, that explains the bluish background, but what about the blood? Unless you supply some light on your own all the photons that light up your fingernails come from the environment, and they are mostly blue. So most will be absorbed and the red will look dark or bluish. This is presumably the reason why so many deep sea creatures are red or pink - it is normally a very dark colour down there without any red light. 
I would assume the sunset vividness of blue flowers would be a combination of this - lots of blue light from all directions - and that the adaptation to lower light levels (the eyes are least sensitive to bluish light and likely start adapting to low-light conditions) make them appear more bright than expected.
See also the excellent Johnsen, Sönke. (2012). The optics of life: a biologist's guide to light in nature. Princeton University Press.
