the main difference between marine and automotive is they're sealed so they dont emit a spark under a hatch.
a 1 wire alternator doesnt actually do anything until the engine gets to a certain RPM and then it makes its own electricity to work the internal voltage regulator. you'll notice as rpm's go up and down that the voltage changes as well.
3 wire alternators have the ability to maintain a 14v constant no matter what rpm. and most HEI ignition systems are going to like 14v more than 12v. most electrical ignition boxes and EFI do NOT like going below 12v.
theres other small differences like the ability to use a charging indicator light, etc. but mainly 14v all times, more wiring needed (gotta run a charging wire, and a switched power wire to ignition), and they're generally cheaper. if you're running stereos and amps and lots of gauges, etc go with teh 3 wire.
