The Indochina region, Indonesia, Borneo, Philippines, etc., Southeast Asia is a very water-centric, and if anything, sort of strike is as a not dissimilar arrangement to the Mediterranean region, replacing that body of water with the South China Sea, the core difference being many, many islands and archipelagos as opposed to the solid, Near East/African coast line.
So by the middle of the first millennium, CE, how well developed was that region in terms of maritime interconnectedness? Was there slight contact and a vague awareness of peoples in other parts of the region? Strong trade routes throughout the various islands and mainland? How influential was China in the trading of the region?
How developed was the shipbuilding trade there at the time? Was crossing from, say, modern Vietnam to the Philippines dangerous or routine?
[Late Antiquity] - [Southeast Asia] - [Ships]
This goes a little past your question about 500 CE but I'll give it a shot. Assuming you don't have JStor access, PM me and I can get you some more resources.
One of the big tropes that's thankfully dying out in regards to Philippine studies is that it was cut off from the rest of Southeast Asia. This is categorically false. The Philippines was one stop on the trade route from China to Indonesia, which then extended on to Madagascar, Egypt, and Persia. This map doesn't really make that clear but it does show the general direction that trade ships would've gone. Now we know that trade with China took place, especially around the time of the Ming Dynasty, because Chinese porcelains were valued as luxury goods all over the Philippines as heirloom vessels for storing rice wine. Visayans also raided the Chinese coast in the 12th century, and were recognized as being distinct from other groups in the area like Cambodians. From the precolonial period onward, they were known as expert boat-builders and the island of Panay was settled by the Spanish in part to take advantage of these skills.
It's also apparent that some Indonesian/Malaysian influence was present in the islands at the same time, beginning at least in the 10th century. The most blatant example of this is the Laguna Copperplate Inscription, written in an Indic script called Kawi and using a mix of different regional languages that was apparently comprehensible to educated people in Manila at the time. This was still the case five hundred years later, when translators working with conquistadors like Magellan and Legazpi were able to speak with local rulers in Malay. Another indication of cultural contact is the existence of the Indian Ramayana epic in Sulu. From what we know of Indonesian history, Luzon was on the very edge of Majapahit influence, which was really straining itself to even control Brunei.
Now the problem for me doing Visayan studies is that none of the precolonial history was written (even though there were scripts like Kawi, they eidently weren't used for historical records), and regional trading powers evidently avoided the Visayas due to piracy. The way I infer this is that Sanskrit terminology is much more prominent in languages of Luzon and Sulu compared to the Visayan languages. Also, we know from Spanish and indigenous sources like epic poetry that raiding was an important part of economic and social life there. It makes sense to me that the avoidance of piracy and subsequent focus on the port of Manila explains in part the comparatively smaller amount of foreign influence found in these islands. Also, it's worth mentioning that the political systems we're dealing with in this context aren't exclusively state-level, but are mostly kinship-based and led by a datu. The Wikipedia article gives a fairly good overview but Robert Blust explains it in more depth.
EDIT to add more sources.
William Henry Scott: Barangay: Sixteenth Century Philippine Culture and Society and Prehispanic Course Materials for the Study of Philippine History
Juan R. Francisco: Indian Influences in the Philippines with Special Reference to Language and Literature
