...I am gonna start by changing the fuel filter
Just last weekend I worked on my buddies 1996 bayliner with the 3.0 LX mercruiser. It had the same problem. It would idle great and try to rev, but bog down and die instead. We purchased a carb rebuild kit for it and installed it. Totally solved the problem and ran great. I would think its in your carb.
That is why we try to find out what engine people have before giving answers. If he has a 5.7MPI engine, your carb rebuild won't help him a bit because he doesn't have one.