being in the wakeboarding section I assume you want to hear it out there and this isn't about surfing.....
your budget is everything with this. You can begin with marine speaker combos in cans and if good enough and amped properly i've heard some sound good. Id say your in the range of 2-400 for the speakers and about 3-400 for a decent used amp.
if your wanting HLCD which can throw the audio your talking $900-$1200 for the speakers, and $300+ for an amp. I have had the Skylon Vector's and added the ARC XDI 804 amp to them and it crushes while out wakeboarding. everything is loud and clear. total cost is about $1300 without wiring if purchased online.