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.
Check the railings as the wood used in these junks is liable to rot. On one cruise (August 2010) a passenger fell about 5 m from the upper deck together with the railing after touching it. Other railings on the ship (Amigo Cruise Phuong Dong 08) were rotted too and had something like formica tacked over the poles to conceal the fact. The plastic lifesavers may be cracked or tied to the ship too firmly to be of any use too.
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.