tlr

After I bought my boat (also 496 HO) I took it to a mechanic for routine service he found one of the plugs was... unplugged.

After plugging it back in the boat ran like 8 MPH faster. So yeah, sounds stupid (I didn't even know it would work at all running on 7 cylinders!) but worth checking.

Raylar

Many times on a Merc 496 engine if there is a stall or with ECM engine stopage the ECM will record a crank sensor and cam sensor fault code in the ECM even though there is really nothing wrong with those sensors or parts. If these codes appear its always a good idea to re-energize the power to the ECM and if needed restart the engine, clear these codes and make sure these codes do not reappear after being cleared the first time with the scan tool to verify that there really is an issue with these two systems. All the early model crank sensors on these engines that had problems were the crank sensors with two O-rings and not the newer single O-ring crank sensor.

Try to remember that good trouble shooting skills and success in fixing engines comes from carefully identifying the real problem or issue and then replacing that part if needed or fixing the issue. Just guessing and throwing $$ and parts at a problem is usually very frustrating, expensive and unsuccessful. "When in doubt, Check it out!" and don't use a WAGMAR (W-ild A-ss G-uess M-ade A-t R-andom) !
Best Regards,
Ray @ Raylar