About the only real problem in cars is the lack of being able to site dead center between the speakers. Other than that, sound deadening can be used to make it extremely quiet. Have you ever been a car with a really well done sound system? I have built, heard and judged systems that were really detailed, smooth and musical. Too many go for the rap beat box systems that are annoying as hell but I always went for smooth and listenable. IASCA is/was an organization that governed car audio competitions and we used Audio Control RTA to check response. One time, all of the RTAs were out at the event so a friend and I (we worked together) had to EQ the system by ear. It scored 37 out of 40 points for that and the imaging was incredible.
Car systems can be amazing and they don't need to be a boom box on wheels but well, not everyone agrees with me on this.
Marine has its own issues. Being outside, it takes a lot more power to make it loud. Also, the Coast Guard has regulations on electrical systems, so a car alternator can't be used in a boat unless it has spark suppression added and this can limit the current output. Space isn't as available on a boat, either and that changes how speakers are mounted, where amps can go without being soaked and where subwoofers will fit. Still, it's possible to make a system sound really good in a boat.
More to the topic of the thread, one of the best sounding systems I ever installed was in a '73 Grand Prix with the red French whorehouse interior. Smooth, great imaging, deep bass but not obnoxiously strong, enough power to make it sound real and it was worth far more than the car. He didn't feel guilty, either.