Oldish thread but might be useful to add a few things comparing the two.
Similan Islands themselves:-
- Granite topography so numerous swimthroughs. Not a lot of coral due to the rock type.
- Although there are a lot of dive boats the sites here aren't TOO overcrowded as there are quite a few sites to spread out (maybe 1 or 2 boats per site at times, sometimes only 1)
- Might get lucky and see some big stuff on the northern sites of Island 9 but maybe not
- Koh Bon in the park further out. Limestone. Good soft corals
- 2 main dive areas, Ridge and pinnacle
- Can be busy as only 2 dive sites (1 really after the first deep dive of the day) especially if big stuff seen
- Chance of Whale Sharks and occasional Manta
- Decent schools of fish and colour
- The pinnacle is a very scenic dive if current and diver levels allow
- Currenty
- Koh Tachai
- Can have big schools of fish
- 2 maybe 3 dive sites so can get crowded
- Occasional big stuff passing through
- Interesting topography
- Sunset dives here can be amazing
- Currenty
Richelieu Rock
- Ridiculously good biodiversity. Things ranging from harlequin shrimp to schools of barracuda and late season, passing whale sharks
- Covered in soft corals and glassfish
- Can get very busy in high season as its a small site
As for
Myanmar, i think best described as "Colder, Darker, Greener".
- Although you can get lucky and get acceptable vis, usually the visibility is significantly lower than Thailand. 6 to 10m of dark green with big strings of plankton, algae and sometimes sediment in the water. Its due to the nutrient runoff from the 300+ islands but dont expect clear or blue waters as a rule.
Further out sites such as Black Rock and Western Rocky can have visibility on par with Thailand if you're lucky.
- Dont expect any big fish or big schools of anything. Mergui is not a marine reserve and fishing boats are absolutely everywhere. The only rule is that cant fish the dive sites *while divers are in the water*. You'll see abandoned nets and fish traps on every dive site.
- Do expect fantastic soft corals and weirder macro life than Thailand (more abundance of Sea horses, frogfish, odd nudis and so on.
- You might get lucky and get to Black Rock and have a passive Manta migration where you'll see tens of individuals for several days on end. However, you might not get lucky and see none at all. Its hit and miss.
- Nobody does the banks that much any more since the shark population got mainly wiped out.
- Quieter dive sites. A lot of sites you'll see no boats. At the highlight sites such as Western rocky or Black Rock you might see 1 or 2 other dive boats only (vs 15+ at Richelieu in high season for example). You will have to share with fishing boats. The image below is the Black Rock dive site to give an example.
View attachment 506541
So for me, Thailand for good vis, a good variety of diving with a chance of big stuff a few times a month. The diving overall even without the big stuff is pretty good around the area but in high season the famous sites can get crowded.
Myanmar would be if you fancy a gamble with MAYBE getting a freak day or 2 of big stuff around and/or you like macro and aren't too bothered by clear, good visibility waters and want to avoid other divers.