This is one area where I am different than almost everybody else. The semi-clear jacketed stuff that the car stereo shops use in boats oxidizes in boats. Marine wiring is simple but different, my first choice in wire is pre-tinned finely stranded marine grade, my second choice is THHN or THWN commercial power wire that is gas and water resistant, my third choice is the stuff the car stereo guys use. More importantly than the type is the size. It's a mistake to size wire based on ampacity alone, you also need to look at voltage drop. When you are only starting with 12 volts a 1 volt drop is very significant for your electronic devices. My audio power distribution block operates at 1-2 volts higher than the stock instrument harness, my amps thank me for using big wire by delivering more power to my speakers.
Here is the stuff I use: http://www.pacermarine.com/wirecable.html
The wire is also approved by NMMA, USCG, and UL.