[cornishfrogboy]

A great white is a rather rare fish. I would be very surprised if a company could guarantee one (Other than Universal when they had Jaws.. and I don't think that this was a real shark in any case)..

Whilst Whalewatching trips can pretty much guarantee Humpbacks and Dolphins, they never guarantee Blues, Finns etc and with good reason. I would put great whites in the same category.

if you go to Jupiter (North of F Lauderdale) you have Florida Shark diving. Much nearer, but state that a Great White is unusual.

In SFO you have trips to the Farallon Islands Greatwhiteadventures.com.
They operate around Sept-November due to Elephant Seal populations in the area and reckon they can come up with the goods! Their tripadvisor reviews are atrocious.