What about seasick on boat?
If you look on Vietnam map, you will see Halong lies on the Tonkin Gulf with lots of island that made up Halong Bay. With its calm water, Halong Bay really acts as a shield that stops all the violent wave, so it's hard to get sea sick in Halong Bay.
Only some of the routes that the junks cruise can be close to open sea that the junk was swinging up and down. But once the junk was surrounded by Halong Bay unique limestone islands, it became stable again.
Tip: When the junk gets swinging, do not walk around on boat to avoid seasick.