there are a few brands i would suggest like rockford, PHOENIX GOLD, JL AUDIO , ALPINE , Kicker, Crunch, Orion----try ebay for any of these, get extra power because you will need it with all the ambient noise in a boat
i'd recomend fosgate, alpine, kicker...ive heard stuff comin outta these and sounded good. i wouldnt recomend summin like sony or pioner...their good brands but their amps aint worth nuttn (trust me...i gotta xplod 760 and it sucks) for bout 300 u can get a good amp from any of da ones me and bob mentioned
if you want REAL amplifiers, go with either US Amps, or Zapco. those are the only amplifier manufacturers that ever win at car shows, but the cost is definately up there. if you want power for cheap, MTX has decent stuff that is generally underrated, and crossfire is the same. stay away from crunch or the like.