If your stereo is located down below in the cabin, why use a marine unit at twice the cost.
Exposure to liquid water is but one concern. While many "marine" radios are no more than car stereos painted white, the better ones will provide conformal coating and other protections that will prolong the life of even electronics that stay "dry".
Consider condensation. What is the relative humidity inside your boat? Does the bilge stay wet with any water? Are you in a fresh water environment or salt? How often do you find that the boat interior goes below the dewpoint of the relatively wet boat interior?
It is the exposure to environmental moisture that is often the bigger culprit. Think salt air condensation sitting on an unprotected copper circuit board trace... It is for this reason that many marine electronics manufacturers will design to exceed testing that mimics fog, mist, and otherwise just damp conditions. ASTM B-117 is one such test, and it specifically is a test that mimics fog on a salt-water coast.
While many sailors can get away with it for one reason or another, (my last boat was completely dry and I am a fresh water sailor) many others will benefit from the extra protection that a quality electronics company will build into their marine radio.
I have used car radios and have done so with good luck, given my parameters. As Stu would say, your boat, your choice.
I will add briefly that installation practice can improve your odds for all electronics installations, be they marine radios or not. This rule should apply to any electronics you install on your boat. Look at the wires going into the unit. I am talking power wire, antenna wire, iPod cables, all wires. Each wire is a potential point of moisture ingress. If your wires come downhill to the electronics, water can and will run down the wire and into the electronics. Use drip loops: If for every piece of electronics you install you routed the wire so that along its path it had a low point lower than the electronics, you will prevent a lot of water from ever having a chance to get in.