Cruising Halong Bay is a nice experience. The Bay is very calm, so you don't need to prepare sea sick pills. Check if your junk (boat) has lifeboat, life jackets, etc. on board and in your cabin. If you know how to swim, it is an advantage. Generally, it is a safe journey for even old people and infants. In the worst situation that you junk sinks and you can swim, if you want to save someone and that person desperately hugs and pulls you deep in the water, you need to go deeper. He will free you, then you can float up to breathe and continue your help.