low rpms on merc 210 hp

My 2003 Challenger 1800 has the 210 hp Mer-cruiser. On land, with a water line, the engine will run regular rpms ( I ran up to 5500 rpm ) However, today, in the water the boat is hard to start then will only pull 2400 rpm. Slow... Will not run at idle (shuts down). Returned to land, same as...