Call the dive operator or look around online, and see how close the sharks will come. 9-18 was a good focal length for me when I did Bahama shark dives, but the sharks came close and were huge (8-15 feet long) so the wide was essential. I would have been out of luck with the 60.
The other lens that might work very well is the 12-50, especially if the sharks are a little farther away. At 50 you are very close to the magnification of the 60 for more distant sharks, but if the sharks come close you can still get good semi-wide shots.
In fact, that lens would be my first choice if you really don't know what to expect in terms of the shark sizes or how close they will come.
Definitely would not recommend the 60. That is a long focal length and if the sharks are so far away you need that lens to fill the frame, your flashes probably will not even reach and there will be greatly reduced visibility due to the amount of water between you and the sharks.
With all big animal dives, wider is better.
Fiji, wow, have a good time!!
---------- Post added August 25th, 2013 at 05:16 PM ----------
I looked up the shark dive online. It is a feeding dive so the sharks will probably be close. The 9-18 looks like the best bet. If you want to keep your distance (not a bad idea on feeding dives), the 12-50 is an alternative. You should bring both lenses and see how the first dive goes. With the Nauti system it is easy to swap ports and lenses.