After quite a bit of research, I decided on the Tokina 12-24mm Lens. Not as good as the Nikon, but very close. So close, there is really no need to shell out the extra €800 for the Nikon.
This pic was taken @12mm from the rear of a 19 foot open boat on the Corrib. I was about 4-5 foot away from the first person. I used a Circular Polariser filter.
Kev