We took an overnight cruise of Halong Bay with Paloma in July. Overall, we enjoyed the experience - Halong Bay is absolutely amazing and not a place to be missed, and an overnight cruise is certainly a great way to experience it. The boat is very nice, the rooms are small but that's to be expected, but they are in great condition and we got a fruit platter in our room was a nice touch. The cruise was pretty packed with activities, which isn't to all tastes, although we didn't mind it, and we've heard that for longer cruises, there's more downtime for relaxing, swimming, etc. The food was very good, not the best that we had during our visit in Vietnam but still good. We were a little annoyed that you have to pay for all beverages, even soda or water, and the boat only provides 2 small bottles of water... so during meals, coffee/tea was provided afterwards but if you wanted anything to drink during the meal (including coffee/tea) then you had to pay for it. It felt a bit like nickel and diming considering that we paid a significant amount for the trip. This may be standard practice among all of the cruise ships but just something we noticed. We enjoyed kayaking around a floating village, and they took us to an amazing cave (although it seems like a lot of different groups do all of this so it may not be that unique to this trip). The drive from Hanoi is 3-4 hours, and we stopped on both ways at what we thought was an irritating, enormous tourist/souvenir trap. Seems like every bus going from Hanoi to the Bay goes there so might be unavoidable. The real draw is of course the Bay, so these little annoyances are worth it, but I'd still recommend bringing your own water/other beverages.
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