The Bay of Tonkin has a 1500 square km expanse of shallow water called Halong Bay in which limestone and dolomite karsts abound. The air is cool and the water is an unreal bottle green. On the day that I visited it was cloudy and the diffused light was less contrasty than I might have originally desired. But it made up for this in spades.
First, because of the weather there were fewer boats in the bay than (I am given to believe) normally ply these waters. Not only that, we were the only ones on a boat that could have held at least 50 passengers. Secondly, the soft even light was perfect for the iPhone 7 Plus. With a dynamic range that seemed made for this small sensor, the little beast was able to manage every scene masterfully. Lastly, it was cool. Cool enough to ensure that we never wanted to go below deck. In fact my advice is to try and choose an overcast day to really enjoy the beauty of Halong Bay. Bright sun, heat and humidity are good for the beach, but not so good for sipping a cocktail and looking out on this geological wonder.
A small piece of advice. When booking a trip, ditch the guide. Just book a car or take a bus to Halong Bay (if you are not already staying in town) and reserve your tickets in advance. The guides do little more than point out some forgettable towns on the way from Hanoi to Halong Bay and make you stop at one or two equally forgettable handicraft factories of indifferent provenance and skill to get you to spend some money that you would be better served spending at smaller village shops or even shops in Hanoi itself, if you are that way inclined.
There are half-day, full-day and overnight boat tours. I personally prefer the half-day tour because it gives you a wonderful taste of what Halong Bay has to offer without ever making you wish that the trip could come to an end. In a span of 5 hours you traverse a distance of some 60 kms, take in one of the most beautiful marine landscapes in the world, eat a wonderful seafood lunch, get to sip your favourite tipple at least three times, and get to visit at least one cavern. What's not to like?
Back on shore, if you are staying the night, there are some great cafes and restaurants that serve fresh seafood to suit every taste. And there's even a cable car ride for those who just need to be reminded that they are on holiday!