I'm sure you've seen photos of Halong Bay's beautiful waters dotted with small islands.
When you're in Hanoi there are lots of organised tours to take you to Halong Bay. I paid extra and went for a luxury option (Cristina Cruise) to spoil myself. I paid $175 which included the bus from my hotel to the boat and then 2 full days with 1 night on the boat with my own double room with a balcony. It was probably the nicest room and bed I've had all year. It was more expensive because I wasn't sharing with anyone and you can get overnight tours for about half that amount but apparently you get what you pay for.
The tour included some caves, kayaking, visiting some islands, an oyster farm and all meals. It was a really great two days and I made some lovely friends but as with all organised tours, it can be a bit frustrating. Some of the group had paid for a second night on an island which sounded great. If you have time, I'd probably make the most of it by doing both nights.
If the weather is really bad (especially visibility) then it will definitely impact your enjoyment.