Rick, as HyLyte mentions, the newer versions of Windows have some great accessibility features in them, and some vendors pack in even more. Start with changing the system defaults to very large fonts, enable the screen magnifier OR THE SCREEN TALKER which will speak the text for him.
Probably don't need anything special there.
I'd also call Garmin and ask them what they know, odds are they've got something in mind for the sight impaired. And, pretty much all of their newer "car" navigation systems speak directions. Some of those can be used with marine charts instead of road maps, that might be the real easy solution.