Well, I think you are talking about the rear shroud being filled with water. There is a sound attenuation gasket at the bottom but it usually does not keep out water. The water will simply find its way out when the boat gets on plane. However, Check the rubber bellows that directs used cooling water out the back of the engine. If this is split it will fill the chamber with water and with the cover on, exhaust fumes will fill the engine, starving it for air.
NOW: All these later Force engines used a semi-keystone top piston ring. They do tend to get caught in the exhaust ports and break. SO: First thing to do is check compression on all 5 cylinders. If it is good--like 140 PSI or even 120, then start checking for spark on all five plugs. Then if that is good remove both fuel pumps and replace the diaphragms. These twin fuel pumps are in series and one bad diaphragm will allow the engine to run but not at full power.
Since you are a new boat owner, you probably don't have a history on the engine. SO: It would not hurt to check the lower unit oil and change the water pump impeller. Where the engine swivels there is a zerk fitting. grease that well and keep it greased to prevent seizing the king pin.
Spark plugs are surface gap and set at the factory. They can be cleaned and re-used almost indefinitely.