Boats leave each morning around 8 am to Muang Khua (5hrs), same time as the boat to Nong Khiaw. If there are not enough passengers you have to share the price of the boat. Boats will also go down river to Luang Prabang if there are enough people.
Boats leave Nong Khiaw, near the bridge every few hours for the 2 hour trip to Muang Ngoi. Try to board the boat early as it can be crowded. i think the fare was 20,000 kip. you will pass by some stunning scenery as the boat labours upriver to its destination.