In fact we went to Bai Tu Long Bay which seems to be the other half of the stunning kast formations rising from the sea. I was told beautiful Halong Bay is definitely busier with many many more boats to contend with. Either way the general area is stunning and it is worth the effort to go there. We booked on the Indochine Dragon Pearl for a 2 night 3 day cruise which was a lovely experience and well executed. There appears to be two identical ships and one does the one night/two day cruise and the other the two night/three day cruise. A short 2.5 hours drive approx from Hanoi in well appointed 4-6 seater vans. No complaints there as we had a hotel pickup and drop off included in the price so didn't have to worry about arranging transport.
We boarded the ship and sailed away early afternoon and then anchored for a swim and kayak session. It really is a beautiful part of the world and reminded me of recent visits to Phang Nha Bay near Phuket Island but especially Guilin on China mainland not too far north of Hanoi - both worth exploring if time allows.
The boats all seem to moor close together in sheltered areas overnight. We had drinks first looking at the lovely views form the top deck and then dinner on the main deck one below outside - which was better quality than I expected, The menu was quite substantial with some lovely Vietnamese dishes to share. The crew were certainly trying very hard to make us happy and sitting on the main deck was a really good experience. They have enough room to accomodate everyone seated undercover if required and use that indoor area both days for breakfast.