I go to Maui any time of year. Between December and April you get the Humpbacks that migrate to calf there and it’s both cool and eerie to listen to their whale song. You’ve can dive Molokini crater, front and back wall, Lanai and Molokai. On the Molokai dive, there’s a good chance of seeing a school of hammerheads. I’ve got friends who live and work there and were just mugged by a pod of dolphins on Sunday, they’ve had encounters with monk seals and whale sharks. You can see mantas on the twilight dives. I’ve seen spotted eagle rays diving the Carthaginian, and turtles on Turtle Reef (well, there’s no actual reef named "Turtle Reef", it’s any spot along 30 miles of shallow coastal reefs).