I'd say there are two choices: (1) have your alternator's regulator fixed at a local repair
shop; or (2) adapt it for an external regulator and get a good one, like the Balmar MC-612 or MC-614.
Even with an internal regulator in your alternator, you should NEVER see more than about 14.4V. These are pretty simple devices, and any alternator shop can fix you up.
An external regulator like the MC-612 would give you more efficient, multi-stage charging and would afford a number of refinements, including:
- setting the profile for the type of batteries;
- setting the maximum percentage of alternator output;
- delayed ramp-up when you first start the engine;
- digital readout of the important parameters;
- easy power reduction when needed;
- attaching battery temp sensors and alternator temp sensors;
Whether the investment in a smart external regulator is worth it or not depends on your situation: how you're set up now, the size of alternator and battery bank, how you intend to use the boat, etc.