As far as physics theory is concerned,

As water is cooled it decreases in volume, and hence becomes denser. a simple example of this when you put water in an icecube tray, and it is almost overflowing, but once it becomes ice the cubes are all separated and are smaller than when they were water...
This means that the molecules become less active and move together, making them harder to separate.

Although there would be a fairly subtle difference between 28 degrees and 14 degrees, there will be a definite difference, hence why it feels like a harder landing, considering the surface area of your entire body.

When the water is colder, you are literally hitting a harder surface, hence it hurts more because there is a greater opposing force on your body etc.

As far as science is concerned, you are not imagining the extra pain.

It is all in ya head . picture this freezing cold say around 2-3 deg than getting a hammer and go to hit something and miss hitting ya thumb you all know you all been there sometime in your life . Now do it in summer same pain but not as intense . Reason is your body is trying its hardest to keep the blood flow pumping around your body to keep everything at a nice 30 deg or so . So with all that blood pumping and you go and smack,crush something yes your head is going to say that hurt more because of the nerves are at there prim waiting for something to happen . So go smack the water and its going to hurt more it is not harder nor softer it is just your brain telling you so .
Was stated some where here that some people go to the beach with a bucket fill it with party ice and water then hold there feet in it for 10 mins . the pros off this is you trick the brain and it goes into "shut down that area mode" and it becomes numb gess what the water you go kiting in now is not that cold now your feet are numb . The effects of this is quite long lasting .

So the the main point of the story get a bath tub fill with icey water and lay in it for 10 mins or more than your whole body will go numb and the water will feel toasty warm and wont hurt so much on the crashes or wait till summer comes .

^^^ all good science Jay but I doubt your body can feel density difference. It is like saying you can tell it is harder to breath on days with certain temps cos air density changed. Even the graph shows that - density diff from 20deg to zero is 0.99 to 1.0. That is nothing.

It would be the simple explanation of pain response in cold temps like surface2air and Skid said.

(Oh and water not being an element..... )

Yep it hurts more regardless of scientific enquiry. Anyone been hit in the head with a footy playing in winter or on the top of your fingers....ouch!

And yeh some cement is in order for sure.

okay i think im getting the hang of this forum business...

Actually, water viscosity changes with temperature !! and a surprisingly amount .

So warm water is softer and cold water is harder !!- think about landing in a bucket of warm honey vs very cold honey - yes your all now thinking about that special person, fun activities and well below the belt I know!

As said above, not sure how the myth busters would go about measuring the small difference - but there is a scientific explanation for everything!!

during the falklands war they found the survival rate of seriously injured soldiers exceeded those that were seriously injured in the 1st gulf war due to the temperature in which they were fighting, so next time you smash yourself up megalooping in winter be thankfull its cold