escher, first off congrats on your upcoming wedding.
To help us help you, how much time do you plan on spending and are you looking for budget accomodations or more fancy resorts?
whale sharks, I think you've got two options for near sure things. 1) snorkel with them in Donsol or 2) dive/snorkel with them in Sogod Bay. Resorts there are very basic. Not very honeymoon-ish. But if you like that kind of thing, both are very peaceful, off-the-beaten-track kind of places.
Mantas, I have only seen them consistently in Malapascua (north of Cebu).
edit: now if you're priority is a nice romantic 5 star resort with spa, then I think Bali might be a better choice. But I'm not 100% sure whether Feb is Manta season.