Do not be dirt cheap when you book a cruise for Ha Long Bay. Horrible reviews a plenty for folks who decided to save a couple of bucks! Some tour agencies were offering $50 for 2 night cruises, horror stories abound.
If you are a party animal, there are the party boats: open bar and a really young crowd. But if you're looking for something laid back, with adults, where you can take in the scenery with a piece of mind, pay a little extra for it. Don't worry about pre-booking, there are a ton of tour operators all over Hanoi that cater to your demands on short notice.
We spent $150 per person, includes transportation in well equipped van. We used Pelican. Highly recommend them. On the way there (4 hour ride, scenic in some areas), the van will stop by a huge warehouse where special needs people create art work sold at pretty high prices. Your call to buy or not!!!
When you arrive at Ha Long bay, you are showered with hellos and lemon tea. As i said, nicest people on earth. Soon you embark on the boat, check into your room and meet in the main dining hall for a debrief. Rooms are spacious, bathrooms clean and well equipped in each room. After debrief, you are served excellent Vietnamese food, and are free to enjoy the scenery.
The boats move at no more than 15/20 miles an hour. You stop to visit a water village where you can kayak for half an hour. People actually do live in the middle of the water. Kinda like a primitive "Water World".
When you come back, dinner is served, again excellent food. You mingle with some of the other guests, you will find them of excellent backgrounds, well educated and traveled. We met a couple from San Francisco, who were seconded to Singapore and exploring the area. We met others from the UK and Germany and we all exchanged valuable travel tips and advice. Also discussed geopolitics in Europe: especially immigration policy and Brexit. Seems like the Brits don't want to leave and the Germans open to accepting people from all over the world. My kind of people.
Go on deck, and enjoy watching the stars over this mystical location. Sleeping arrangements comfortable. Woke up early to catch the sun rise over the islands. Not to be missed, even with cloudy weather. There was a Tai-Chi session on deck. Didn't partake, but enjoyed watching the captain, who doubles as Tai-Chi master, lead the guests into strange exercises and poses.
After a hearty breakfast, you head off to explore the caves on one of these islands. Not the best caves i've ever seen, but worth the long line and the crowds. Come back to the boat, head back to shore, and drive 4 hours back to Hanoi.
Word to the wise: don't be cheap!