Good Evening (well, I guess that it's tomorrow for you down in Oz)....
Using a CPL on the 12-24 is a bit different than with other lenses. CPLs generally, do not work very well on the lens (and we are talking in particular about the sky), because its focal length. Beyond about 24mm, the lens is too wide for the polarization to really work across the entire field of view. It can certainly still be used, however the polarization effect will be concentrated in one area. If that is the effect that you are going after, then it will work just fine.
In terms of removing reflections from water and other reflective surfaces, then it acts just fine. It also adds saturation to colors.
So, with that out of the way, I actually do have a CPL for my 12-24. I picked up a thin, low profile one just so that I would either not have (or mitigate as much as possible) the potential of vignetting. I picked up a Nikon PL2 77mm. It was not cheap, I think that it ran me a bit over $100 (also its the same size needed for the A* 300/f4) and that was a number of years ago. Nikon no longer sells filters - but I do have to say, this does do a stunning job. It is sufficiently thin enough that I have not experienced any vignetting with it. And, yes when used with sky shots, I do get the non uniform application of the polarization (and have felt that I ruined a few shots using it). You need to really evaluate the shot. Also, the 12-24 does do a fine job in terms of contrast and pulling the colors. Depending on the shot, having the reflection off the water can add to the image and other times detract from the image. If its a shot that you really do want, I would shoot it both with and without, so that you can decide afterwards what turned out better.
Hope that helps....