Engine starts great. The boat and engine runs great for a short while then after fishing for an hour or two I start the engine and it starts but there is a beep. Helix 9 shows low voltage on the display. While running after this the engine start to shut down. It will idle but has no power. After getting back home I check the voltage at the battery and it is only showing 12.4 with it running. I changed the battery and the alternator and with it running it still only shows 12.4 at the battery but it show 14.1 at the alternator. I cannot find a voltage regulator anywhere on any diagram. Is 12.4 at the battery when the alternator has 14.1 at the red wire normal? I haven’t been back to the lake since changing the alternator so I don’t know if it will happen again or not. Any help out there.

Trace the wire to insure you are measuring that voltage at the two ends of the same wire. You don't need a diagram to do this. If it's the same conductor, you've got some high-impedance defect. A wire from the outboard to the battery isn't going to have a voltage drop like that. Do you have a battery switch? Are all the 12V/GND wires & connectors clean (no corrosion or tarnish) and tight? If you do, check on the outboard side terminals on that switch. Be sure to trace the wire to look for points where a problem is happening. BTW, since you still have a problem, it's possible there was nothing wrong with the alternator and battery you removed. Did you check that alternator's output before replacing it?

My thought is it was the battery and is probably good to go now. Voltage Regulator built into the alternator.
https://miamialternators.com/Mercury-Outboard-200-HP-Optimax-Wp-Alternator-8471/

Everything is clean at the starting solenoid and the alternator. Going to make a jumper to try and see if that works. Long battery cable is the lat resort I guess. I checked the voltage at the battery with the old alternator but not at the alternator itself. There was black dust and metal shavings on the old alternator

Your original message said: "....I changed the battery and the alternator and with it running it still only shows 12.4 at the battery but it show 14.1 at the alternator." That says you are reporting two vastly different voltages at the two ends of a heavy conductor, which can't happen without some issue between the two points. I guess you've changed your story with the latest post. I'm still thinking only the battery was bad, but that's been replaced now, so you are likely past any trouble. The Mercs really want a hot battery or they get fickle.

Ok. I am not a mechanic and I am a beginner with a multimeter. So yesterday I traced the red wire from the alternator to the starter solenoid. It was all clean the best I can tell. It is inside a bunch of wires inside a plastic sleeve. There is nothing else in between them unless it fits inside the plastic sleeve. I checked continuity and it showed good. At the solenoid it connects with 4 other wires with one being the cable from the battery. Continuity showed good between the solenoid and the battery. While I had all the wires disconnected from the solenoid I cleaned them again with a wire brush before I checked continuity, again all checked good. I reconnected all the wires and started the engine. I let it run for about 10 minutes and checked the voltage at the alternator. It showed a reading of 14. I then checked the voltage at the end of the red wire at the starting solenoid and it was just 12.4. I checked the voltage at the battery and it showed a 12.4. I am guessing that says there is something wrong with that red wire between the alternator and the starting solenoid. I will try and replace that today and see what happens. If that fixes it then yes I probably bought a battery and an alternator that i really didn’t need. That means I shouldn’t have to replace them again in the near future. If anyone has any other words of wisdom I am open to anyone that can help. Thank you