If you plan to visit here, slow boats are the best choice. You can come from Luang Prapang or use nice down stream and start from North Thailand. Travel first to Chiang Rai and go to Thai counterpart to the Lao border in Chiang Khong, then with boat (10 000Kip; 6am to 6pm) to opposite side of Mekong were is Lao custom clearance in Huay Xai.
Huay Xai - Ban Pak Beng (night) - Luang Prapang takes two ways and I propose you to take a package with boat and accommodation, you get it easier and don't need to worry availability. There is two night packages also.
There are multiple providers the web link is one of them and only for example, I have not used them.
95000 kips from Hoay Xai to Pak Beng and another 95000 kips from Pak Beng to Luang Prabang, with each took 6-8 hours. The boat carry some 100 passengers, mostly backpackers. Some tourists gathered at the back of the boat to enjoy 'grass' and Beerlao 15000 kips per bottle, for the entire 6 hours, an international grass-beer party. Some seat-less passengers prefer free-sitting, like me, I rather walk around freely and play card games with some tourists.
If you are not sure about the boat ticket, you can buy it at any travel agency in town in Hoay Xai and Luang Prabang, they charge 220000 kips (bargain if you want) for the 2-day boat ride.
You can also buy a boat ticket 95000 kips to Pak Beng, and buy again the bus ticket (via Udomxai) when you arrived at Pak Beng, flexible.
Speed boat ticket is available too, cost much more than slow boat, you can get it anywhere if you show your money.
We took a boat from LP to Huay Xai, stopping overnight at Pak Beng.
The Day 2 boat was of concern from the moment of sighting - and we were all openly unimpressed by the size, unstable seating and general condition.etc.
Very early in the trip, the boat broke down - and drifted into rocks - but was restarted. We were hours overdue, when the boat broke down again - close to sunset. The driver was not able to restart the boat. After a further wait, another boat arrived, into which we clambered, the boats were lashed together - and we were taken further up the river. We were unceremoniously dumped in the middle of nowehere, somewhere near Huay Xai. There was a 90 degree bank to clamber up - and we were told there was a driver at the top - 20Bt each to Huay Xai. The boaty disappeared.
After the bank clamber, we were greeted by a sonthaew driver, who told us the fare was 40Bt, and there were unhappy people who took off. Negotiations proceeded, and more people took off in disgust at the whole predicament. The driver was increasingly aware that his fare was diminishing rapidly. He eventually agreed to 20Bt - after at least 20 minutes. As we were about to leave, 3 older people from the boat emerged over the bank - who might just as easily have been left stranded if the argument has not gone on.
We ended up safely taken to Huay Xai, and somewhat banded together, but all quite horrified at the lack of accountability of the boat owner.
Some people had prepaid for other connecting travel from Huay Xai, which they missed, and one couple's visa expired that day, and they were relying on crossing to Thailand that night.
My advice to other travellers would be, if you think the boat is in dodgy condition, wait for another one.