Boats ply the Mekong to and from Huay Xai at the Thai border, stopping in Pakbeng where you can catch overland connections towards the northeast and the border with China. The trip takes 2 days by slow boat, or 6 bone-rattling hours by speedboat. There are also operators now offering 2-day "luxury" cruises.
If you have the opportunity, purchase a pillow from a local market before embarking on any boat ride that lasts longer than 2 hours. Expect to spend the night in Pakbeng if you're taking a slow boat (the safest option), or to arrive in Huay Xai deaf, shaken and either exhausted or exhilarated from three hours in a speedboat. There is also a twice-weekly "one day comfortable boat" between Luang Prabang and Huay Xai, but the cost is significantly higher.
Slow boats leave every day, usually around 8-9AM. The trip to Huay Xai costs 220,000 kip (Dec. 2008). If you can, just purchase your tickets at the boat landing because all the tour agencies in town charge a commission, and agents usually don't have reliable information about the quality of the boats. It is not uncommon to have to switch to a new boat in Pakbeng, so you may end up in a boat of higher or lower quality for the second half of the journey.
The slow-boat is generally packed - so much so that there may not enough seats to go round. Arriving early will mean a longer day, but most likely a better seat, towards the front and away from the engine. Earplugs are recommended, regardless of where you end up sitting. Travelers report that those who show up better-dressed may end up with better seats.
If you choose to travel on the speedboat (a light canoe with a very powerful engine), a crash helmet and life-jacket should be provided - it is not recommended to travel in a speedboat without this essential safety equipment. It is also recommended that you make your bags as waterproof/water-resistant as possible and wear a rainjacket - the boat can generate quite a bit of spray, plus any small showers you might encounter along the way will sting like needles against any exposed skin. On sunny days, sunscreen is invaluable as there is no roof/shade on these speed machines. The journey to Huay Xai can be reduced to as few as 4 hours in the wet season, with a lunch stop at Pakbeng. However, some consider this means of transportation less safe, especially in the dry season. Earplugs are strongly recommended. Travelers who are concerned about creating as little environmental impact as possible may want avoid speedboats, as they are heavier polluters than the slower options.
The third option is to take a "luxury" cruise. The major operators are Luang Say (http://www.luangsay.com) and Nagi of Mekong (http://nagiofmekong.com). As of 2009, both operate two-day cruises to Hauy Xai that stop in Pak Beng for the night. Although the journey takes as long as taking the slow boat, both operators offer vastly superior facilities and equipment than public slow boats, and you should be prepared to pay a premium for it.
There is no public boat service to Vientiane, but it may be possible to do the trip by private tourist boat when the water levels are high enough.
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