Up to now, Deep Water Soloing is definitely the best climbing activity in Ha Long bay. An enchanting and mythical landscape, a feeling of delirium, no worrying and wondering  about corroded bolts (bear in mind that most of the equipped routes in the Bay are not climbable anymore, the bolts being badly oxidised and dangerous to use, except the latest SloPony openings in Cat Ba and...

some other sectors  (see Ha Long bay topo).

Things to remember

Carefully choose your spot (inclination, depth, solidity and feeling of the rock). Double-check the depth. With the turquoise waters being very opaque, take care to mind the shallow reefs. Read the tide table - the higher the tide the safer most spots are, but use your judgement.
Ha Long Bay comprises of more than 3000 islands, most of them being barely suitable to DWS (most of them are shaped like an hourglass, with a base expanding below the surface of the water). In order to help you, some ideas are provided in the DWS Topo section.
If you find some new interesting spots, share your discoveries! Vietclimb and the climbing community will be very grateful to you. While a full and thorough topo may be a long time coming, a basic one made with everybody's experiences, will continue to be constructed and should serve the purpose sufficiently so that you can get out there.

When ?

Despite the widespread urban legend about tropical countries and year round summer, it is actually really freezing cold in winter in the North Vietnam. DWS is impossible (from late December to late March generally), unless you have Viking blood running through your veins or are fond of rolling yourself naked in powder snow.
DWS is possible and highly recommended the rest of the time.

"I look at climbing not so much as standing on the top as seeing the other side. There are always other horizons in front of you, other horizons to go beyond and that's what I like about climbing."
Chris Bonington