I guess that sums it up. The main reason is diving, and it is great for that - when we went, we dived with Sam’s Dives, who I can recommend. They took equally good care of me (qualified diver) and my wife (non-qualified), and there are loads of good easily accessible divesites.
Other things to do: snorkeling and kayaking (loads of little islands to explore) were both fun. Jellyfish lake is quite famous - the only (i think?) stingless jellyfish which you can snorkel through. And the dolphin sanctuary is supposed to be good too (you can swim/dive with them) - we didn’t go as it sounded slightly cheesy, but people we spoke to said it was really good.
Things not available: Nightlife and beaches in any large quantites.
I think flights there are cheap now, but it’ll still probably quite expensive on the island.