There are a huge number of domestic connections, particularly to the ports of Halong City and Haiphong.
Independently from Hanoi
There are 4 main ways to transfer from Hanoi to Halong Bay:
From Hanoi, you can take a mini bus from Gia Lam bus station. Tickets are bought at the ticket counter for 90,000 VND. The ride is approximately 4 hours long, and the bus will stop to pick up passengers along the way. Air-conditioning is minimal, so be prepared to break a sweat in summer. Larger sized tourists will not find it to be a pleasant ride, but it is a nice authentic Vietnamese experience.
By open bus:
US$15 one way, pick up & drop off at Bus Station
By shuttle bus: Pick up & drop off passengers in the Ha Noi old quarter. 4-6 hours pending on how many stops en route the driver makes to pick up/drop off passengers. Fare about 10 US/ way.
By Train:
take cargo train from Hanoi to Yenvien and then to QuangNinh. The terminal is about 50km away from the harbor. Expect train delays and cancellations without notice. Hard sleeper only, US$10 one way.
Organised tours from Hanoi
There are many package tours to Cat Ba Island from Hanoi, Standards on these vary considerably: while comfortable - or even downright luxurious - boats, excellent food and knowledgeable and enthusiastic guides may be available, most are characterized by long and crowded bus rides, tourist-oriented rip-off trips on boats so slow you could swim faster (to get you to buy more food and drink on board), hard-sell 'add-ons' like brief stops for kayaking, 'water cave' visits, and other shady practices. So if you must visit Cat Ba, simply avoid the tourist port entirely
