I hope it's simply a charging system problem. Look into repalcing just the brushes and regulator on the alternator, rather than the whole alternator, if possible. On some, the whole job takes 5 minutes without removing the alternator.
Something tells me it might be more than just a charging system problem. A couple of hours at anchor should not discharge a battery. Perhaps something else is drawing lots of current and the alternator is struggling to keep up, hence the low voltage.
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