This may not be universally or near universally true, but most of the "marine" head units I've seen are not water resistant. They require an enclosure for protection from washdowns, if not the rain.
The ones from major manufacturers are UV- and corrosion- resistant and have tinned wires.
So, if you aren't concerned about moisture or salt air or the plastic (especially white) changing colors and becoming brittle from the sun, you don't necessarily need a marine unit.
I've seen plenty of car radios in boats. I'd have to look, but I think my galley radio is ~10+ years old and not marine. It was installed by 2 or 3 owners ago and the wiring all looked good when I relocated it. I've also seen plenty with corroded connections, knobs and buttons not working quite right, or that have given up the ghost, mostly older ones.
When I've replaced them, I've always used marine units. I just try to avoid non-marine stuff in the boat when there is a good marine solution available. But, that is me.