Does anyone have information about travelling from Dresden to Prague by boat?

I don't know if it is possible to travel by boat all the way from Dresden to Prague. I know that steamboats go from Dresden to Decin (the first town in the Czech Republic). A few km behind Decin (near Usti) is a dam which might make it impossible to travel further ... but as I said, not sure.

The river cruising ships that I know only go as far as Decin as well. From there they offer transport by railway to Prague.

Sorry that I cannot be of more help..

Boats certainly can go from Dresden to Praha: up the river Elbe (Labe in Czech) to Melnik, then up the river Vltava. Upstream of Usti, there are locks.
However, there are no regular passenger boats upstream of Decin. Most cruise ships are, I think, too big for the locks. For regular trips between Dresden and Decin, see www.saechsische-dampfschiffahrt.de who have a famous fleet of paddle steamers. The biggest boat operator in Praha is PPS (www.paroplavba.cz) who also have paddle steamers; they do occasional trips from Praha down to Melnik where the Vltava joins the Labe.
Dresden-Praha is an easy journey by train. The railway follows the river very closely between Pirna and Decin.