Yes they can. Generally the wheels are retractable so they do not interfere with the smoothness of the floats. There have been many original aircraft over time, and also many conversions.
One of the issues is the pilot getting mixed up with what sort of landing they are making, for the normal warning of no gear down when in approach configuration is not appropriate. A Cessna 206 amphibian conversion I flew (not handling takeoff/landing though - a special skill and licence is required) in the US, which had wheels fore and aft on the tips of the floats had a voice warning that cut in at 200 feet which announced either of "Wheels up - water landing", or "Wheels down - runway landing" depending on whether wheels were up or down.
Careful attention is required to wash the landing gear with fresh water after sea landings to avoid salt corrosion of the landing gear mechanism.