Hi, do you have analog or Smartcraft gauges?
What is happening is the ECM is not detecting water pressure and is limiting the engine speed.
Water pressure is checked by a dash gauge, Smartcraft gauge or the diagnostic program or DDT scan tool.
Are you getting a warning horn as well?
A quick check is to go to the transducer on the back of the engine. Look at the center top of the flat cover in the back. The water pressure transducer has a hose on one side and a bunch of wires on the other side.
Remove the hose from the transducer/sensor. Have a helper start the engine and see if you have water coming out of the hose. If so OK. Stop engine.
Check the wires from the transducer for any broken, pinched etc. If the wiring is good you may have a bad sensor. You can get one from your local Mercury dealer or online at discount from Boats.net
Also if you do have low water pressure the engine can be heating up and not allowing the rpm's to advance. This could be does to a weak water pump, open poppit valve etc. If the water pump has not been replaced in the last few years I would do that first.
If the boat is operated in salt water there could be other blockages as well.
Let me know what gauges you have, warning horns, and check the water flow. Post back and we will go from there.