The challenge with a single DIN is there isn't a great way to secure the unit both to and behind the dash. Another issue is the current marine covers are all too large to fit the 'stock' location on the dash. I also didn't want to spend a ton of money. I decided on the Fusion MS-RA205
, list is $249 but I think I found it on sale for $199. And, you don't need to buy a cover (which won't fit the boat anyway). It has a USB input on the back. I just use an Apple cable run into the back and keep the i-device under the dash nearest the driver. It syncs with my ipod to show the playlists, etc. It also has decent FM reception and is compatible with Sirus. I did have to cut the opening slightly, and picked up a trim plate from a car stereo shop as a filler, b/c the Fusion is slightly taller, but more narrow than OEM unit. I can post pictures when I get home tonight. For speakers I went with Polk (don't remember the number) marine certified which were available locally. The Fusion could benefit from an amp - but it's fine as is. It's survived a couple of bow dunkings where a tidal wave came over the windshield, rain, etc and works great.
I 'almost' went with a marine amp along with an ipod idaptor - which is another good option. Especially if FM/Sirus doesn't mean anything to you.