how many amps? how much total RMS? i wouldn't mess with your alternator unless absolutely necessary. i'm running 2 amps rated at total 1500W RMS with the stock 03 70A alternator and two optima blue tops and so far it doesn't look like i'm going to need to change my alternator. it would be better, imo, to add a 2nd or possibly 3rd battery and isolator along with a good muli-bank smart charger that you could charge the house bank off the water. good marine grade alternators are pretty expensive. the balmer 150A units that some here run cost over $800.
2003 X-2 MCX