As I know, only coaches runs from Jinghong to Laos. The bus leaves Jinghong Bus Station at 06:40 everyday, taking 6 hours on the way. The fare is CNY120.

There is a boad leaving Jingong every second day 8:00 am (monday, wednesday, friday and sunday sometimes). Price 850 RMB. You could go out in laos.
Problem1: for "westerner" you need laos visa which is not provided at harbour.
problem2: ferry service strongly depends on water depth. if no rain for a while, no boat will go. sometimes they only decide the evening before leaving.
solution1: go take bus as suggested by other user, you will get visa at the bus-crossing border for 30 US$.
solution2: take the boat down all the way to thailand. chiang-saen. you can cross the river there and be in laos.