There is a common misconception in this thread.
Extra batteries will give you more playing time. If you alternator is not keeping up the extra batteries will just delay the time until they go dead.
1700 watts is really not that much. I ran 1500 off my old boat with a 60A alternator and one starting battery and it never went dead. However, I never played it unless the motor was running.
Most boats require about 20-30amps to run the ignition and boat electronics. The remainder of your alternator amperage is available to power the stereo. Assume that you will average 50% power draw at a 50% duty cycle -> 425 watts average draw. At 14.2 volts that 29 amps. If your boat has a 60 amp or better alternator you should be fine with no additional batteries. If you want to listen to music while parked add an additional battery(s) to run the stereo.
I would also suggest you add one of these for cheap insurance, it's like an accurage gas gauge for your battery: http://www.lascarelectronics.com/PRODUCTS.CFM?STOCKNO=EMV%201200&CFID=22736882&CFTO KEN=49933503
Don't get lights, be careful with impeller pumps too, both are big current drains.