I won't comment on component selection, you have already got some good advice...but for salt use I can comment on some other items.
Interconnect hardware...Use QUALITY stuff. Quality wire from Monster or Street Wires. Power dist from them as well.
Battery connections should be CLEAN, as well as amp and speaker connections. This includes ALL Power and Ground connections in the boat. Dirty ground connections are a big cause of Alternator whine, especially at the engine block.
Solder connections (If you can) then shrinkwrap where possible too.
DI-Electric is your BEST FRIEND!!! This will help keep the salt-air from starting the corrosion process, and help in resist rusting.
Understand, you CAN'T get away from corrosion in a 100% saltwater environment. I know, I have over 1000hrs on my boat entirely in saltwater. It's all about the maintenance.