You can still travel by cargo ship between China and Thailand, but it is limited to 2 foreigners per ship and takes some 24 hours, as they go slow and anchor for the night along the way. This method is basically catch as catch can. The captains wait until they have a decent load then sail. So it is not something you can book in advance or check availability for.
I don't think the faster boat has resumed the service, that was stopped after the two crews were killed on Chinese freighters a few years back. This was run by Mekong Delta Travel in Chiang Rai and last I heard all their boats were still offered for charter service only.
Note: my information sources are people in the travel industry, not easily found on the web.