Another tidbit for folks wanting to design a Southeast Asian-inspired setting: Sundaland, SEA's real 'lost continent.' Before the end of the last glacial age, when sea levels were much lower, Peninsular Malaysia, most of what is now Indonesia, and even some of the Philippine Islands like Palawan were connected to the continent of Asia as one landmass geologists now call Sundaland.
Sundaland is the reason why these islands have continental Asian flora and fauna such as deer, tigers, rhinos, elephants, and water buffalo, while Australia and New Zealand don't. Anthropologists speculate that the ancestors of Australia's Aborigines made their way to Oz via the coasts of Sundaland, marching or canoeing until Australia was only a short canoe hop of open sea away.
When the glaciation ended and the seas rose, Sundaland was inundated leaving only the highest points as islands. The larger mammals died out on most islands, which is why the Philippines' biggest wild mammals are wild boar and deer; tiger fossils were found on Palawan dating back to c. 15,000 years ago, but eventually there wasn't enough big prey to support them.
As for Man, scientists are still wrangling over whether the Austronesians who populate Maritime Southeast Asia came from Sundaland, from the north by sea through Taiwan, or diffused from the Malay Peninsula by sea.
Now we get to the interesting part: Could there have been one or more civilizations on Sundaland? According to most histories, Mankind didn't even know farming yet, and the only domesticated animal we had was the dog.
But new finds are pushing the boundaries of history ever farther back, and many advances like agriculture and larger permanent settlements seem to have occurred much earlier than scholars first thought.
Still, the idea of an advanced civilization that early is unlikely. But of course that's what we gamers want! So yeah, for our purposes there was a Lemuria.
Let's assume that the inundation of Sundaland was pretty gradual. Marine biology findings seem to bear this out.
I got to attend a lecture by underwater photographer Lynn Funkhouser, who showed slides from a recent biodiversity study comparing numbers of species in Australia, Hawaii, and the SEA Coral Triangle (I live smack dab inside that Triangle, whee!).
The Coral Triangle had the most species by a huge margin, and Funkhouser said the scientists now believed this was because slowly Sundaland had isolated marine life into many lagoons until sea levels rose high enough for them to get out and mix.
So Lemuria could've existed as a peninsula of Asia, but like the northern Mediterranean it would've had a very 'squiggly' coastline with lots of bays, gulfs and lagoons isolated from the sea or even fully landlocked. Sinking was pretty slow, noticeable over several generations -- maybe it did so in periodic floods, aided by quakes and volcanic eruptions, instead of a constant slow sea level rise.
Plenty of time for a civilization to develop. If you go for a more 'realistic' feel this civilization could be similar to Mycenean Greece, broken up into rough city-states with seafaring economies. If you like a more gonzo feel, speculate away -- you can always explain the sea as having hidden everything interesting.
After all, it's Lemuria's coastal plains and valleys that now make up the floor of the Coral Triangle, so every place mankind would've settled is now underwater and well-covered up.
The idea of Sundaland = Lemuria gets really interesting for me when I consider that the Lemurians could've been Austronesian, or the ancestors of the Austronesians. That gives me a reason to mash together Malay and Polynesian elements, and if I knew more about the early history of the Malagasy I could include that too.
Big stone temples like the ones in Ponape. Scowling gigantic eidolons like those of Easter Island. Epic sea battles on catamaran dreadnaughts. And spirits more powerful than anything After the Flood!