I was at Halong Bay in mid Feb. and the weather was cool and rainy, definately not swimming weather, but ok for kayaking if you don't mind drizzle and mist, which is typical for Feb. Despite the weather, I am glad we went, but one overnight on the boat was enough for me.

Ditto shelley ks comments above. We were on the bay one night in second half of February. We kayaked and got wet and didn't freeze to death. It was gray, misty, and cool. But still looks beautiful. Our guide mentioned that a person or two had swum in his group the day before - but they were clearly young and hardy. . .

Thanks for the feedback. In your opinion, would we be better off going to Sapa (which I understand is also quite cold, but I would rather have cold when I am trekking than when the main attraction is chilling on a boat) or sticking with Mai Chau and Halong Bay?

I did not go to Sapa or Mai Chau during my trip because I was advised by others that the weather would be cold and misty in Feb. I think the weather doesn't really get better in these areas until mid March at the earliest. Perhaps others can give you first hand feedback on the Feb.weather in these areas.