Do the accommodations have to be honeymoon like? Do you want to do a liveaboard?
December is sorta like the off season of whalesharks in the South West Pacific for some reason. The only places I can think of that have good hits for whalesharks in Dec are in the Indian Ocean.
The Maldives are great for honeymooners with a plethora of wonderful resorts with chances to catch whalesharks and mantas to boot. The South Ari Atoll is one of the areas they lurk. Check for liveaboards or resorts offering snorkel/diving cruises.
Djibouti/Kenya and to a lesser extent Tanzania (Pemba, Zanzibar, Mafia). Djibouti would be on a liveaboard and you get both mantas and whalesharks around the area. Viz isn't the best in the world but that's why the plankton feeders are there.
Kenya/Tanzania would be from resorts. Kenya's better areas would be off Diani, Malindi and Mombasa. I like Pemba and Zanzibar in Tanzania for romantic getaways (done it a few times), Pemba being the least luxurious of the 3 places.
For something closer to Oz, you could try X'mas island off WA since the crabs will have dumped all their eggs by then. I can't remember any nice accommodations there though and certainly don't consider it a romantic place.