Re: car stereo instead of marine stereo(saltwater)
No other than possibly having dust/water caps for the RCA outputs a marine stereo has the exact same wiring harness as a non marine counterpart. The wiring is done the same, 12v constant, 12v switched, ground, and speaker outputs. They also have remote turn on for amplifiers and such, pioneers and a few other brands usually have a dimmer wire but 90% of people don't even hook it up anyways.
Many marine units do have the option for an external wired remote, ie one mounted below decks or at the swim platform. I THINK some sony radios support a wired remote that plugs into the changer port but i'm not a big sony fan.
If you go look at a car stereo you are paying for features, normally the mid to high range have the exact same power output as the base models, the only difference is the features such as reading MP3 cds, USB inputs, and number of pre-outs.
As far as marine stereo's go they will withstand much more humidity and water splash, some even use stainless steel cages and frames. They will warranty a marine cd player that goes bad in a boat where as if they find water in a non-marine unit your warranty is void.