The boat will not stop at the caves as they are north of LP. You have to take a tour from LP, or go by public transport or motorbike to the village across from the caves and arrange for a boat to take you across the river. Although the caves are worth seeing , they will be very crowded. I think it would be best to visit late afternoon as most of the tour would have returned to LP by then
I believe the private boat companies - Mekong Smile, Shompoo, Nagi of Mekong, Luang Say - make a stop at the caves as part of their itinerary, but not the Public boat.
Britchick is correct - the private boats (Nagi, Shompoo etc) stop at the caves and at various villages on the 2 day slow boat trip. The cheaper public boats do not make the tourist stops. All slow boats stop overnight at Pak Beng.Edited: 7:37 am, May 10, 2018
I’m booked on the Ngai this coming November, they make several stops along the way including Pak Ou caves and overnight in Pak Beng.
Awesome. Thanks for the info, everyone. I visited Pak Ou back in the late 90s and want to take my wife there.