My question related to the lower case "r", of the term "GPSr" which I now surmise means "receiver". Sorry for the confusion.
One sublety in the nautical/marine world is that buried in the bowels of the Vista HCx programming is something that swaps the device over to "marine" which means that it doesn't force one to take an embedded (included, loaded, pre-programmed, ??? road). I guess that means that courses followed are as defined between waypoints, until the travel commences.
I have a lot to learn. I'll have to call Garmin to get the low-down on WAAS in Belize fringe area.