Try www.palm.com for a start. They have a searchable software data base. Some of the other software and shareware sites also include Palm software.There are one or two programs around that will interface with a GPS. A good one is http://download.cnet.com. I saw an Astro Navigation program there.
Let me know if you find anything particularly useful.
I found three navigation programs there plus something doubtful that's supposed to turn your handheld into a compass when you attatch a magnet to the bottom (as if!!)There's also a program that allows you to monitor your progress uder handicap during a race.
Try searches on "navigation" and "boat" for starters.