The problem is trying to find a waterproof unit that fits in that very narrow 2.5" slot provided. Most marine stereos need at least 4 inches of height for the cover. Some guys have put in "Fusion" stereos but they were more than I wanted to spend ($350ish).
Here is what I did, but in doing so got a lot of flak from the "don't cut into a classic crowd". There stereo ran me under $100. I also put in an amp and some new speakers.