Very much like the much smaller but still spectacular Phang Nga Bay in Thailand with jungle-draped limestone islets and their scarps jutting vertically from the sea, cave systems, floating fishing villages and attendand tourist flotillas.
Given the 4 hour drive from Hanoi and a desire for a more than perfunctory visit we opted for a 3 day, 2 night stay aboard the Jasmine run by the Heritage line, the same company that operates the Jehan, our vessel for the Tongle Sap and Mekong in Cambodia, both boats having a well-equipped, wood and brass old-world charm. Both are big enough to provide for spacious facilities yet small enough to navigate bays and lakes and limit passenger numbers to a sensible level.
Mangroves and seagrass beds have been cleared, jetties and wharves have been built for tourist boats and game fishing is threatening endangered species. Floating detritus is common, likely from mainland run-off but dampening (no pun intended) any desire for a swim, even in the oppressive heat. This is a UNESCO World Heritage site and hopefully tighter environmental management will stop this still beautiful place from being degraded any further.
A don't-miss-it is the Dau Go Cave, with 3 large chambers of impressive stalagtites, stalagmites and shawls. A long, steep climb in and out but more than worth the effort. My photos did not come out well - here's an external link should you have any speleological interests.