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If you have an internal regulator in your alternator--that's no good for boats, they are designed to "not overcharge". An external regulator is designed to "charge quickly and fully". They are different pieces of equipment, the internal one is cheap, the external one costs more and does more.
To amplify what Cam is talking about: Alternators aren't always matched to engines, either. Let's say that your engine idles at 500rpm but you cruise at 2500 rpm with a "battle speed" of 3800rpm. There's a pulley on the engine that you aren't going to change, because it costs a lot. There's another pulley on the alternator. The size ratio between the two of these, determines how fast the alternator will turn at any given engine speed.
So, if the engine pulley is 15" diameter, and the alternator pulley is 5" in diameter, the alternator turns (15/5) 3x faster than the engine does. At engine idle speed of 500 rpm, the alternator turns 1500 rpm. At that speed most alternators put out next to nothing. At 2500 engine rpm, the alternator is turning at 7500rpm, typically a good spot for nearly full output. But, ooops, at 3800 engine rpm the alternator would be running at 11,400rpm which would burn out most alternators after a short while.
So, the maker typically will install an alternator pulley which won't let the alternator burn out--and reduces power at idle and cruising speeds even more!
The solution to this is, first, to get an alternator with a wide RPM range. Some can put out full power at 10,000rpm (alternator rpm) with a working range of 2500-15000rpm or higher. Cheaper alternators usually have less effective range.
Then, second, once you find out the specs on your alternator, consider having a custom pulley made up in a machine shop if the one you have does not properly match the speeds. Boat builders "buy stock" because it is cheap. A custom pulley will run you about $100 from a machine shop, but it can double the output from your alternator if what you have wasn't properly sized to begin with. A VERY effective fix, if that's all it needs. But you've got to run the numbers to find out where the problem really is: pulley, alternator, regulator. They all have to be matched to each other and the battery bank size, to work well with each other.