Hi notaguru,
Welcome to the forum.
Fugawi (
www.fugawi.com ) specializes on maps for lakes, rivers, and
oceans. The Fugawi Marine ENC Ver. 4.5 GPS navigation software could be what you are after: http://www.fugawi.com/web/products/f...marine_enc.htm , but also check other Fugawi's products on their website.
The above is about US $210 for just the software, though. A GPS receiver would be another around $60 - depending on which one you buy.
You could also just try perhaps the least expensive solution - Microsoft Streets & Trips. The program is not designed for such purpose (it is a trip planner for USA/Canada), but it does include the whole world. You would be able to view your position on the map no matter where you are. It only depends how much detail you require. MS Streets & Trips will not zoom in very much outside USA/Canada. The maximum detail you could hope fore is shown on the screenshot bellow:
If the above detail would satisfy your needs, then Streets & Trips can be purchased for less then US $40, or you can get the version which comes with the GPS unit for less than $100.