BINGO and if this was a 555 ecm / pcm it would be toast. 1000 percent about that. MEFI ecms hold up a bit better but again all ECM / PCM s like steady proper voltage from start up to shut down with no spikes. The hate low voltage and they hate high voltage as well for a jist of info.

ALT has 3 wires, Orange, Red/Purple, Purple and may also have a black connected but not always but it is ground

Orange and Red/Purple should have Battery voltage full time whether ignition is ON (motor running or not)
Purple will be 12V only when the key is ON (motor running or just turned on)

With a voltmeter
measure the orange at the ALT post and at the battery and they both should be the same
measure the red/purple at the ALT and it should be battery voltage.

Turn key to ON and see if the purple wire goes hot

If all good, start the motor and get rpm up above 1500 rpm just for a couple seconds then let idle. Measure voltage at the bat, you should see 14.5V

Alternator is good but it measures 13.4 volts at idle. I ran the boat in the driveway for about 20 minutes but as the engine came up to temp I noticed the engine wasn't running very good. It will idle at 600 rpm but soon as I give it gas it sounds sluggish. Revs but sounds like it's missing a cylinder or two. Closer to the running temp I noticed my oil pressure at idle was 0 so I shut it off and checked the oil. Oil is a quart low. I put a quart in and started it back up. I had oil pressure about 19 psi and when revved 30 psi. Instead if fighting with the interior in I decided to take all the interior back out so I can do a compression test tomorrow. I pulled all the plugs and noticed everything on the right side of the engine looks lean/dry. The first three plugs on the left side look very carbonized and even wet and the last one on that size looked leaner dry. Any chance I have fouled plugs? Maybe so much fuel while running causes it to hydrolock?

If im looking at a oil pressure problem how will this engine respond to low oil pressure? I've read that there is a guardian mode but will it shut down the engine and stop it from restarting?

Btw there is no lifter noise or other odd noise at idle. If pressure was a problem I would expect I lost the bottom end or lifter noise. Can a oil pressure sensor cause the engine to not start or try? Im going to replace the oil filter, do compression test and maybe change the plugs. Maybe my oil is too thin? Using the merc oil 10w-40.

Been following this, can't wait to hear the end finding.
My opinion: If oil pressure read 0 hot idle first thing you need to do is install manual gauge and verify reading. Checking compression will be good information but I don't think will tell you anything about this problem. If oil pressure is really 0 at hot idle then the engine is coming out anyway. Remove the oil fill cap and smell the cap and in valve cover. If you have ever opened an engine up that has spun a bearing the crank case smells like burnt oil, very distinctive smell. If you take the oil filter off cut it open and look for metal. 13.4 on volt meter wouldn't concern me at this time, maybe after running correctly. I've seen engines sieze and then turn over after cooling, but it is usually obvious there is a problem. If it was fuel pressure related the engine would still turn over. If it was due to low voltage it would not fix itself by sitting for two hours. Good luck Cableguy. I think you have a winter project.

Being 1 qt low on oil will not cause oil PSI issues. Maybe sender, gauge, wiring, etc., Best to check oil PSI with a mechanical gauge.
Did U scan for fault codes- no other choice with EFI engines and gives a good starting page for diagnosis
compression and also add leak down check too
replacing plugs alone will not help unless U find out why they are black/carbonized
But, my suggestion since your skills are limited and replacing parts guessing with out diagnosis it's be best to have a knowledgeable find the issues.
I'm a retired "Euro" auto tech and have seen over and over owners "guessing and replacing parts".
comments only reflect opinions

OK more thoughts: Sorry if I missed this, but have you checked your drive oil? Not just level, is it burnt, is there metal on the magnet? Last thing you did was change prop. Is that all correct?
Only thing I've seen make oil pressure go away and then come back is sludge/crap in oil pan. As you run it clogs the pick up screen and starves the engine for oil. After shut down awhile sitting the debris falls away from the screen and you have oil pressure again on start up until it clogs again. Good luck.

This was my thought to. I fear I have a spun bearing but let me tell you a little about what I did. First off I changed the oil to 20w 50 oil and new oil filter the replace the spark plugs with champion equivalent of the mr43t plugs recommended. I checked compression cold and warm and no compression problems. After I started it up it sounded good. Clean idle no weird noises. I checked spark to every plug with no problems. Fuel pressure looks good. Now oil pressure warm is 30 at idle. On the dash. Idled for 30 minutes in the driveway.... no outdrive on. Alternator tested, battery tested, all spark plug wires, and even starter was good. When I rev it it sounds like a detonation in a cylinder as it's coming off the rev I can hear a knock of sorts. Doesn't sound exactly like a rod or main bearing but maybe. Once its back at idle smooth and clean. Almost like it's missing a cylinder or ground cam lobe except it sounds good at idle. 600 rpm. I would take it to a merc dealer to put it on the computer but with them wanting $200 to pull it in and put it on the machine I could have bought a rebuild kit for $600 but if I buy a rebuild kit and it does this again I'll be pretty upset. Thinking of taking it back to the lake and attempting another run.

If I have to rebuild it she will get head work and a cam.

Is there any kind of limp mode on these engine the kills some cylinders after so many rpms?

Your engine does not have a limp or reduced power mode

For info about --- MEFI 1 for the first 3 years had guardian mode or per the OEM and industry it is also called POWER REDUCTION MODE ACTIVE.

and MEFI 1 was installed up to 1998 but the later half of MEFI 1 was POWER REDUCTION MODE WAS NOT ACTIVE

Engine serial number is how all this goes by -- YEAR MODEL means zero

All the mefi 1s with PRM - was the largest made ecms. The last half of MEFI 1 s were smaller in size and those did not have PRM