I have a 1997 pontoon with the mercruiser 3.0lx inboard with stock alum prop,the boat has 80 hrs on at present. The problem is that it will only turn 3500 to 3700 rpm's at wot. I've replaced plugs,cap,rotor and wires and still no help, the motor seems to have a large amount of vibration at idle. Water in gas?
Wouldn't go that route, if it used to reach WOT with this prop, the answer is not lower pitch.
Compression check and fuel contamination have both been mentioned - also both good suggestions. The 3.0 doesn't come with a water seperating fuel filter - now would be a good time to add one.
Is the vibration only present/worse when in gear? Is the prop in PERFECT condition or is it nicked and dinged?
Cruise on up to the adults only section and read about all the reasons an engine won't reach WOT. While you are there download the manual for you engine and drive - they are must haves if you will be doing your own work.