Not as much help, but my new wife and I rented the Rohendi (their luxury yacht) for a half a day to go snorkel with whale sharks in late June and didn't see any. They were supposed to be in season and we thought we'd have a decent chance of seeing one. On the plus side, the guides were great, the food was good, and we did do some very up close snorkeling with manta rays. The down side - it is bloody expensive. They have other options, such as a ski boat sized speed boat and a dhoni, which are much cheaper, but nowhere near as comfortable.