Ningaloo in April?

[LR]

Go for Coral Bay. I can vouch that it is an amazing place. Regularly you can see and swim with Turtles/Reef Sharks/Rays and Manta Rays and then also see Dolphins and Dugongs and Tiger Sharks and then Humpback Whales in season as well. All in a 5 hour boat trip. The Reef is right there, 10m off the beach and there are no long boat rides involved. Whale Sharks turn up there earlier than in Exmouth usually. It does costs a fair bit for a Whale Shark trip but 95% of people say its worth it. You are going at the best time of year.

For Whitsundays, try Islandive.com, good fun young team, great boats and know there stuff.