In reply to Short&Savage:
Went to Ha Long last May, its out of season so its not so busy and the tides aren't the best but we still got plenty of DWS done.
We chartered a boat from Cat Ba Ventures which unless theres a few of you is quite expensive at about $450 for 3 days/2nights.
Our local Vietnamese guide was excellent (Victor) but be careful leaving your money laying about, we found one of the boat crew going through our bag one night.
There's also Asia Outdoors located in Cat Ba who use to be Slo Pony, the blokes running the climbing were American/Australian. They arrange DWS trips, you'll stand more chance of meeting other climbers with these lot and share the cost.
I loved Ha Long but if your on your own could be better to stick to stick to Thailand in May, there wasn't too many climbers in Ha Long to meet up with randomly.