An almost identical situation occured to me.....
Just got the boat (454 Mag MP)I) back from a valve job.
Was this situation ever solved?
If so, what was the problem?

Haven't licked the problem yet-- checked the ignition timing (per the above procedure) and it looks pretty close-- but the rpm does not rise to 1,500 like it's suposed to when I jump terminals a & B-- so I reved it manually to there and the timing marks advance to where it looks decent. I checked the ecm codes-- nothing, just the one long flash and three quick ones over and over... Also pulled the distributor cap-- just a bit of cap terminal corrosion-- nothing to account for so much lost power. I'm going to buy a fuel pressure test guage and see if the regulator or fuel pump has pooped out-- but I think that may be a long shot. I'm thinking about pulling the plenum and checking the injectors-- but the manual's procedure for checking them seems kinda iffy. I'm at a loss, really. Guess I'll have to take it to a dealer.... $100 an hour labor, here I come!

I've lost 7 mph from a bent prop that looked ok to the naked eye.My cruise speed went from 50 mph at 3000 to 43 mph. Changed props and good as new.
Now add a dirty bottom and you could lose some more. Perhaps not one problem but a combination of a few .

Does your prop have growth on it or damaged?Just a thought!Always check the basics before surgery!Another thing,when the motor was rebuilt from the overheating,was the overheating problem repaired?Over heated valves may not close all the way when under load and the tick would scratch my head!

Might want to look in any hidden compartments for water. Might have seeped in if it sits in the water. Pull inspection plates and check. Saw a boat that had a compartment full of water where the motor mounts are through bolted.

Good ideas, one and all! I'm sure the props and drive are coated with slime as well-- it's a B3 so a lot of prop area. I have no signs of water anywhere, but that's a good point. The heads were done with the rebuild (100 hours ago) and I saw them being delivered from the machinist so I'm sure they were ok then, but who knows now. When I checked the distributor-- other than having the cheapest looking aluminum post cap on it-- it was fine (I bought a brass post cap-- before I found out that there is very little difference in performance between the two). I think I will just cease diagnosis on the whole thing until I take it from the slip in two weeks-- then get the bottom cleaned, and go from there. Thanks very much for all your help, and I will complete this thread once I get a resolution- maybe it will help somebody else...

Going from 57 to 35 mph and it appears to fire on all cylinders. I would definitely check fuelpressure at WOT.

By the way, did you find out why the engine overheated to the extent that you had to rebuild it after 150 hours. Bad fuel pressure is a not unusual cause sice you also had detonation. You must check it at WOT and it should be 43 psi with the vacuum hose disconnected from the fuel pressure regulator. Likely causes; clogged fuel pick-up, anti siphone valve, fuel filter, fuel pump, fuel pressure regulator or possibly injectors.

By the way, did you find out why the engine overheated to the extent that you had to rebuild it after 150 hours.

Yep--- owner before me trashed the sea water pump and his mechanic hadn't chased down all the pieces- I had to clear the oil cooler passages with a coat hanger to dislodge them, then back flush. Blown head gasket, two hosed pistons-- one dished inward from detonation and one acutally had a chunk out of it on the edge! I will buy a fuel pressure guage and check the pressure tomorrow. You say check it at WOT-- I know this is a dumb question, but would that be in gear while moving or just out of gear revving the snot out of it? Is it safe to run these engines 4-5,000 rpm with no load on them?
Thanks again for all your help.

Most of the fuel starvation problems only show up in high demand situations. Therefore you want to check the pressure under full load ie in the water, in forward gear at WOT.

With the vacuum hose to the fuel pressure regulator disconnected the pressure should be rock steady throughout the powerband.