Check out Balmar. They specialize in Marine high output alternators. If you can get me some numbers off your current alternator and can find out what it currently is and maybe come up with some alternatives.
Another good thing to consider is that you may have to change out your cables as they may not be equipped to handle the extra amperage.
Do you keep your boat at your house or somewhere where there is a 110AC power supply? If so you may want to consider putting in a dual bank automatic charger and charging the batteries every night.
Is your alternator not keeping up with your system while you driving around or is it not picking the batteries back up after partying with the boat off? The alternator is not designed to charge batteries. It is meant to maintain the batteries and support the electrical systems on your boat. If you drain the batteries a bunch while sitting partying and then expect your alternator to charge up the batteries you are going to burn out the alternator.
Don't cheat yourself on the alternator. Don't go buy some $150 POS from China that says it puts out 150 amps. Also when considering Balmar be careful. Just because they are marine does not mean they are UL Coast Guard approved and safe for use with a gas engine.
I know Grant on here put in a 200 amp Balmar with an external Regulator. I have questions if that alternator is truly safe for use with a gas engine, but maybe he had them modify it to make it safe for gasoline use.