Whenever a client wants to get better charging from an engine(s), my first recommendation is to make the best use of the DC-generating capacity you have. Swap the internal (automotive style) regulator on the alternator for a smart three-stage regulator. Most alternator shops will modify the stock alternator for a nominal fee.
Since you have dual inboard engines you'll need a three-stage regulator that can drive two alternators at the same time. Most three-stage regulators don't have sufficient amperage on the field voltage connection to drive two simultaneous alternators. AMPLE Power, based in Seattle, has a regulator module (DAC) designed to drive dual alternators.
Upgrading to a higher-output alternator is a common next step. The alternator needs to be properly sized to the specifications of the engine manufacturer. Note that a larger output alternator may be limited by the alternator bracket and the belt size.