Either way, having a unit that plays MP3s and has an auxiliary
input will not be difficult. Just about everything sold today has an
auxiliary input.

The major cost is the adapter. I found several adapters wouldn't let me use
the iPod's control wheel while attached to the adapter. This was a major
downside for in-vehicle use. The Neo Ion lets me use the Ipod controls
along with the controls on the head unit:

http://www.neocaraudio.com/prod-ion.html

Do you have transom and cockpit controls already? If so, use that as your
starting point. Or if you want to add them then make sure whatever head
unit you're considering will support them; along with a player.

If we get the Four Winns 348 Vista I'll probably consider adding an iPod
adapter. I'd greatly prefer one that allow track changing. I don't need to
see track names on a display. Just being able to skip to the next or jump
back a few tracks. The Clarion unit supposedly limits you to using just 5
playlists. Not exactly what I'm after.