......up to twelve days after the full moon, from memory.
SeaHorse Dive Shop are the founders of this trip and excellent. He books out quite a long way in advance so be organised. Do a search for their web site and it has tons of detail, and videos even. I've been down three different times, each time with them, and over the six days have seen them every day with Brian - when we got back most people had seen blue water. We were Diving which has a better strike ratio, but when you do find one they will come to the surface, so snorkelling is good too. To get the best chance of a sighting I booked two days running. The Last hour or two of daylight is the most productive time - if you get a choice - as I believe they give operators time slots now, but I presume that happens on site rather than when booking.
Nothing can prepare you for the awe of a forty foot fish brushing you as it swims by with an open mouth that a car would fit inside.