Boats or ships

Cruise ships float with help from density and a principal called buoyancy.
because of greater surface area.

sometimes

The ships are built in drydocks which can be flooded to float the ships out.

Usually it was because the rivers were used to transport materials and goods. Saw mills were very popular way back then and many cities grew around the mills because that was were the work was. They used the rivers to float the logs and ships to transport other things. Eventually railways were used in the same way, and then of course trucks.