Hoffa...sorry but that is not right. Wet cell batteries can only take about MAXIMUM of 20% of capacity during bulk charging which means that a 60amp altenator is fine for a 300ah bank.
Even with charging system loss...fully charging is simply a matter of time. Incomplete charging can happen with ANY size charger and is definitely a problem if you don't monitor what is happening in your charging routine.
He does not need a bigger or better alternator cause the boat will be MOST of the time at the dock and the charger will put on a proper charge and EQ when needed. Nothing bad is going to happen to his batteries in a week of cruising with a standard regulator even if he doesn't pull into a dock. Why spend an extra grand on a big alternator/regulator and install if you don't need it?
AGM batteries can take unlimited current and a 100amp range alternator would be more appropriate for those if a quick charge was needed. (Note: Going too much over 100amps on most sailboat engines causes bearing wear and creates a whole 'nother set of problems to deal with to insure that wear does not occur...otherwise you could add a 300 amp alternator for AGM's if you weren't worried about the horsepower loss!)