US and US-ally naval officers have told me the big fear in southeast Asia is terrorists hijacking an oil tanker. The terrorists blow up the tanker and create an ecological disaster. Of course this attack could be executed anywhere. Straits (choke points) channel sea traffic. Pirates in small, fast boats can launch a quick attack from the coast. A terror attack will always generate huge headlines, but the headlines will be bigger if international shipping is stalled because a key strait is closed.
He has much more on maritime threats.