Old post, but... the CW is this is indeed the best time to go. We're going out this year with Nautilus Explorer.
I;ve spend a lot of time in the water with sharks in many areas of the world, and I prefer a boat that has an experienced crew, does not chum and bait. The concept of lots of chum and chunky bits flying around, sharks coming in with one thing on their mind and unnaturally stimulated behaviors just turn me off. I'd rather observe natural shark behaviors in their environment. I've done some research, and read Undercurrent of course, and for my money these guys fill the bill.