Alternator upgrade opinion?

Hey guys,

It's been a while since I posted on here as it's been a long hard winter here in Michigan and the boat was in storage. I am trying to put the finishing touches on my 78 so she's totally solid for long days on the water. Since I got her there have always been electrical drain issues over longer periods on the water. I installed a Sony Marine Stereo system with 4 infinity 6x9 speakers in the boat, and I'm not sure if that is the issue or irrelevant (alternator can't keep up with the juice demands of the stereo?). Yesterday I was out for about 2 hours with a buddy of mine and she ran well other than a slight hesitation (that we think was just dirty plugs that will be replaced). The hesitation was getting better as we drove it, but the ride went long as we started with a full tank and were trying to see if driving it would clear off the plugs and smooth out the hesitation at higher RPM. After a while I tried a hole shot after idling for a bit, and she died. My buddy tested it and it was 10.8 or something, not enough to even turn it over. So we brought it back and charged it, ate dinner, and took her back out. She was fine but this is a theme that has to end. I need her to be reliable and not leave me stranded!

I have read on skidim that older boats with (this one has a commander marine 351) alternators with external voltage regulators, should be replaced with internal regulator models for reliability and cost savings should the regulator fail, as somehow internal regulated models do less damage to other electrical components (I assume they mean starters etc)?

I also have a buddy who has a ski tique and he took his to a marine store that is closed down now (or I would go ask them). But he got some kind of high power output alternator to ensure he was going big enough when replacing his OEM factory alternator. I was wondering if there is a possibility that there is something to upgrading my alternator and if it's reasonable to assume that the old one is just shot?

When testing the battery under way at about 1200 RPM or so, the battery was only showing 12.7 V and my buddy and I think this is just too low for a working alternator. It should be at like 13+ when under way shouldn't it?

I'd like to ensure that the battery is very well charged so it turns over as this big 351 doesn't exactly crank like spinning a plastic top!

Let me know what you guys think and I hope you're all having a good summer!

Jared