I have a 1999 OX66 3.1L that has a loss of power. The boat just plows in the water and will not get on pad, it seam to me that its not firing on all cylinders as it shakes real bad. once the boat finally breaks over it will only top out around 50ish right at 5500 RPM

I have cleaned the 02 sensor popit valve and replaced the gaskets, replaced all 3 pumps, new plugs,cleaned the VST filters, change 1 coil, replaced both head gaskets, compression test on all 6 are +/- 1 or 2 PSI from 120. fuel pressure test maintained 30 PSI in load range, changed primer bulb. Also drained the VST pump to check for water in the fuel, no water.

After doing all of that, there still no change and im lost as to where to start next.

No expert but I've seen where cleaning a bad o2 sensor doesn't always work. Also what about sparkplugs?

^^^This^^^

the red spark plug covers are resistors and 1 or 2 could be bAD DID YOU USE THE CORRECT PLUGS

of course i used the right plugs. I didnt clean the 02 sensor just the popit valve. Never thought about the plug cap, ill ohms them out tonight.

Carb motor ?

High speed jets need cleaned

The OX66 is fuel injected.

Injector cups , don’t the little screens get plugged up on those ?

Or could it be a tuner issue like on mercs when the bolts back out and the tuner drops ?

Clogged exhaust can cause this.

Clean the O2 sensor. From your description of the symptoms the O2 sensor fits the bill.

If 02 is oil soaked it may not clean I've taken carb clean and sprayed it done then use a propain torch and headed it up to burn it clean.

Replace both manual fuel pumps ...All that’s wrong most likely

The low pressure pumps won’t cause that motor to not run once out of idle. They will make it hard to start and idle but it will run normal. once you get up and going.

I just recently replace all 3 on a friends OX66. Diaphragm in top one blew out while we were running across the lake. Motor started bogging and lost a few hundred rpm's on top. So yeah it can.