Not sure what he used but I would say roughly 300mm.
In Dahab for instance, if you shoot from the shallows in the lagoon, the doughnut or baby bay then shorter may even be best and up to 250 will get some good shots if you choose your location well.
Thank you! I finally went for the 'el cheapo' 55-250 IS.
The gallery of what it can do is at photo.stihii.net/hu-may-2009/index.html
(I should've had more attention to composition at shoot time, but please dont be harsh on me - I'm an amature