An extension of the same reef system that includes the Similans in Thailand, scuba diving around the islands that lie over the border in Myanmar waters is equally impressive. Just as full of small fish, possibly more pelagic creatures and equally pristine corals, the only reason these dives are less well known is due to the political situation in Myanmar.
Politics aside, many Thai liveaboard operators have special permits to cross over to the Mergui Archipelago and it is well worth seeing the reefs there. It's not that it's so different, with the conditions, visibility and so on much the same, but these reefs get far fewer divers and that makes it feel like there is so much more to discover. The most exciting site in this area is Western Rocky, a large pinnacle that is reminiscent of Richelieu Rock to the south – and yes, there are whalesharks there, too.
The other big attraction is the remote Burma Banks, which are well known for their shark populations. Although targeted by illegal fishing from time to time, the authorities have managed to gain some control and you are almost guaranteed to see nurse, silvertip, black and whitetip sharks. Most boats will conduct a controlled shark feed: you may not agree with this, but it one of the rare moments when you get to see these apex predators.