1

I think it's a win-win situation if you're choosing between those two. I lived in Belize for several years and absolutely loved it. The diving was incredible and my time spent out of the water was great as well. Check out Glover's Reef, the Blue Hole (accessed via either San Pedro or Caye Caulker) and Placencia. Awesome diving and snorkeling! Hope that helps :)

2

I spent a solid three two weeks diving in the Seychelles last year and it was lovely, but I wouldn't call it world class. Indonesia and Borneo were both better experiences for me, as was Egypt. It was very expensive as well, an example was a day trip we did from Mahe to La Digue and Praslin cost us $500 for two people. We also found that we were a bit stuck for activities outside the water as well (We were in Beau Vallon on Mahe). From my perspective the islands seemed very well equipped for the all-inclusive resort style holiday where you relax by the pool all day and less able to cater for the independent traveller. Depends what you're after I guess. We had a great time and the diving was enjoyable and really easy, but I think you'd get more for your dollar in Belize.

3

Is the choice only between Belize and Seychelles? I've not been to either place, but my recommendation for a honeymoon in the Indian Ocean with access to some great diving and snorkelling would be the Maldives.