[ReeferJon]

Thinking about this problem for a while... at the moment (unless you use the old Newton) there's no "Apple" way of doing this, so it them becomes a choice between Palm and PocketPC. A lot depends on the format of your marine charts.. if their MapTech, then I'd suggest [url=http://www.memory-map.com/index.html]MemoryMap or Pocket Navigator.

For the PDA, go for a PocketPC running Windows Mobile 2003 SE (eeuugghhh), preferably with VGA resolution and bluetooth (ie. [url=http://www.totalpda.co.uk/HP-iPAQ-hx4700-Pocket-PC.aspx]HP iPaq hx4700

Now, if you're on-board, you can either "mirror" the Mac screen using something like VNC over Bluetooth or WiFi, or, I believe, stream GPS data from your Mac to the PocketPC using Bluetooth (you should be able to repeat the data to the Bluetooth port) and use MemoryMap to display your charts locally.

If you're walking, you'll need a GPS unit, either CompactFlash or Bluetooth.

Oh, and you'll need some software to install Maps / Apps on the PocketPC.. either VirtualPC running Windows, or Mark/Space's software, or PocketMac.

Not cheap, but it should work...

If anyone's had any success running a VGA-displayed PocketPC with VNC and GPSNavX, please post!