Just back from the JadeSails cruise ( http://jadesailshalong.com ) after the recommendation from this forum, and highly recommend it. As others have noted it's a day cruise, and that's really what you want to do.

Pickup 7am from your hotel, very very fancy luxury bus (think massage chair built into the seats) takes about two hours to get to the port. Then you have eight hours on the cruise, and two hours back by bus. Brand new European style yacht, rather than the 'traditional' junk. Excellent food and staff. Cost for me was ~$100 for the cruise, $15 for drinks on board, and $7 for a tip.

A couple things of note that weren't clear to me before I went:

- 'Ha Long Bay' is actually two 'bays'... Ha Long Bay, and Lan Ha Bay. It's all the same body of water, and all looks the same, but they're controlled by two different provincial governments. The province controlling Ha Long Bay is very restrictive, and they only allow your boat to stop in a small handful of spots. Effectively, what that means is that all the boats cluster together in one area, and it feels over touristy; not relaxing at all. The province controlling Lan Ha Bay is much more flexible, and although there are some rules about where you can stop, it's pretty much open anywhere. This allows the boats to spread out a lot more, so while you do see other people, you're not all 100 feet away from everybody else. TL;DR: you want a cruise that goes mostly to Lan Ha Bay.

- The reason you want a day trip and not an overnight is because most of the overnights spend time doing crap like 'visiting an authentic Vietnamese fishing village'. Well... you and 8,000 other tourists, and it's pretty clear the village's primary industry now is tourism, not fishing. We sailed by that on Jade, and I'm glad a stop that wasn't on our itinerary.

- We had 26 of 70 (capacity) on our cruise, and it was perfect. Many other boats seemed similarly empty. I don't know the seasons well enough to know when boats are packed and when they are not, but I'd definitely try to find a time when they're not. (I went mid Oct on a weekday.) All the operators still seemed to be operating, but you just get a lot more relaxing experience.

Highly highly recommend Jade Sails -- after looking at every other boat out there, it was clearly the best. Plenty of heads turned from other tourists as we sailed past. I booked direct, but booking through a hotel or local tourist agency can probably get you a bit of a discount.

If you do want an overnight, the same outfit runs a different boat, Stellar of the Seas: https://www.stellaroftheseas.com/ We sailed past it at one point and it looked ah-mazing, so I'd look into that. But honestly, just do the day thing. Overnight isn't worth it.