Depends what and where. Some folks backflush with the water hose if i's something removable.....piece of your old impeller if damaged and blade(s) broken off not caught in the water pump repair (a possibility, not necessarily the norm), weeds, running in a sand bar and picking up a lot of sand possibly. If running in salt water and not flushing after use it could be Barnacles, salt deposits that would require the water jacket covers to be removed from the block at the rear where the spark plugs are located and the flat cover on the left, looking forward from the rear, side of the engine. Another option would be to drop the lower unit and force water up the water tube that goes up into the bottom of the block with the thermostat and popoff valve (both under the same cover on the plate you removed that surrounds the spark plugs.
The Pop off mentioned above regulates the cooling above 2500 rpms and that valve could be stuck and not opening properly, or the flow path through it could be blocked. It is closed at the lower rpms and the thermostat adjacent to it controls temp there. Opening temp on my 90 was 143F stamped on the pellet part of the device. 143F is about the temperature of a US domestic hot water heater set to Normal for an idea as to how hot that is. Since water boils at 212F at sea level, there would be no steam if working properly.
Others could help more with this. I never had a clogged OH problem. I did have a thermostat stuck closed once but that caused the warning horn to sound at the low rpms where it is supposed to be controlling cooling. Once the boat got up on plane and rpms were high enough, the Pop off valve opened and allowed for proper cooling and the horn shut off.