Halong Bay is amazing and completely worth the time and effort to plan. Having said that, do put some time and effort into choosing what tour you do, as this will make a big difference to whether you come back raving about your trip or warning others not to make the mistakes you did. Here's some advice I wish I'd read before I started planning.
There are literally hundreds of boats floating around Halong Bay every single day, catering to all interests and budgets. But to find the one that caters to you, you need to know what it is you are looking for. That means:
1. How much do you want to spend? Of course, everyone's budget is different, but I recommend spending a bit extra and going for a mid-range boat. Remember you are going to be stuck with it/on it, spending a little bit extra for a bit of comfort, good food, is probably worth it.
A tip: If you like a bit of luxury, consider upgrading. We chose a mid-range cruise and upgraded to a suite which I personally thought was better value than the basic cabins on the more expensive boats.
2. How long do you want to go? There are basically two options here: 2 days/1 night or 3 days/2 nights. In my view, you absolutely should go for the 3 day/2 night option, you won't regret it. If you do go for 2D/1N, be aware that you will spend less than 24 hours actually on Halong Bay. The first day you will make the four hour trip from Hanoi and arrive at the boat after 12 noon, after spending overnight on board, you will then disembark at around 11 am before making the four hour trip back to Hanoi while asking yourself why you didn't go ahead and do the 3 day tour. On most boats the 3 day tour does the same itinery as the 2 day tour, with an extra full day added in the middle where you will get the whole day exploring some of the further out areas, away from the tourist hordes.
3. How many people do you want to go with? How many people do you want to share your tour with? In my opinion anywhere up to 30 people is good, but I would steer clear of the huge boats.
4. What do you want to eat? It may sound silly, but you are going to be stuck eating what they give you for two or three days. The quality of the food will largely be determined by your budget, however there is quite a range of options depending on the boat. Some are predominantly buffets, however there are others (like ours) that serve you individual courses, or even ones where you can choose your main course from a menu. This is especially important if you have special dietary requirements. I've read so many reviews from non-seafood eaters or vegetarians saying that, although they specified their requirements when booking, there were very few options for them. The Boy doesn't eat seafood so this was a key factor for us when choosing a boat and I have to say that the team on our boat did a brilliant job of providing a variety of tasty non-seafood options for him.
5. Where do you want to go? In broad terms there are two basic options: Halong Bay (to the west) and Bai Tu Long Bay (to the east). Both are spectacularly beautiful, however, Ha Long Bay has many more boats giving you many more options, whereas Bai Tu Long Bay has less boats making it less busy and less crowded. Many people also claim that the Bai Tu Long Bay is cleaner. If you go to Bai Tu Long Bay you will miss out on seeing some of the more famous caves but you might get to visit some more untouched caves. Most boats provide a description of where they go on their website so check it out. We opted for Bai Tu Long Bay because we didn't fancy the idea of being shuttled through various sites with hundreds of other tourists, and we were very pleased with our decision. If you'd prefer to go to Bai Tu Long Bay, check out some of the boats listed at the end of this post.
A third option is a tour that spends one night on Cat Ba Island. These tours are generally less expensive than the other options, however, be careful to check out not just the boat but also the accommodation offered on Cat Ba to make sure it fits your requirements.
6. What's included? Do you want to go Kayaking? If so, many boats charge extra. Does the price include transport from Hanoi and pick up from your hotel? Also, depending on the boat there may be extra activities offered on board such as cooking demonstrations or squid fishing. It's worth finding out exactly what's offered and what extra charges there will be.