Hi my name is***** to JustAnswer ! I will be assisting you.
Just a couple of questions;
What are the Engine Operating Hours?
Do you know the service history?
Have you experienced or come across and other engine related problems or performance related problems since the boat was last serviced?
Have there been any alarms come on since the last service or occurring now?
These are just a couple of things that can really help in troubleshooting your problem that you should be aware of before we start to possibly get into lengthy troubleshooting.
Just going on what you say you might be experiencing with your engine, we can go with that and first look at the IAC. That is the Idle Air Speed Controller. Lets check the filter! It's located inside the Throttle Body or on the Port side of the throttle body. Let me know the Serial Number of the engine and I can let you know which one it is. In the mean time, I can send you a pic of what the filter looks like and the location of the filter in models with filter inside Throttle Body.
Check how bad the filter is blocked, replace it and we can take it from there.
Make sure the spark plugs are replaced at around 300hrs.
Let us know how you go with it!