Every time it rains...my CJ leaks water on top of my radio. So I took a rectangular plastic lid and cut it to size. Now it lays on top of my radio inside the dash and doesnt get wet.
at my local boater's world the only sealed enclosures they had would only seal the front of the unit. they don't do anything for what's behind the dash. they might make one that encapsulates the whole radio but i haven't seen them locally yet. the marine cd players are nice, and a sony can be had for about $200, but they look nice... which tells me that it will be stolen long before it ever gets a chance to be tested by water.
Just be sure not to cover the gill that's usually on top of the units. They allow heat to escape. If the heat can't get out you'll have a whole new batch of problems.
Mine got wet and shorted out the radio but the cd player still works. It will only play store bought cd's though and won't play at all if it's really cold or really humid.