It was in the 2000s. People from history are on a boat. They don’t know how they got there. Mark Twain is one of the people on the boat in a sparsely populated natural-setting type world. There is an evil bad guy chasing them. There are also the beings who created the world as some kind of test.
That’s all I got. I don’t remember what network, who was in it or whether it was a regular series or a mini -series.
In an attempt to be more helpful, I’ll point out there was a made-for-cable movie version of Riverworld in 2003 (it was intended as the pilot for a series but the series wasn’t made) and then a four-hour reboot miniseries in 2010 (which again didn’t get picked up as a ongoing series). Both were aired by the Sci-Fi Channel.
No, no, no! It has to be a World of Piers. You can’t have a river of tiers without using channels and locks and pumps and stuff. Maybe some sort of siphon.