While I hope your GPS idea does work, I'm afraid that the typical R/C'er might find that it isn't too practical for use. Cost is one of those 'impractical' things, something in the neighborhood of $300 - $400 just isn't a normal part of my R/C'ing budget to get a boat back to the bank...without financing by that bank (pun intended).
Accuracy is still not scaled up enough even with a shore beacon. Nearest one to me is a bit over 100 miles away. Considering the terrain between it and me, and the typical behavior of UHF signals, the GPS I use locally has no idea the beacon even exists.
- 'Doc

Off topic part.

I 'play' with a system that uses GPS for position reporting called APRS (connected to another expensive hobby I have). I track my position using a computer and a GPS receiver to furnish information. It's accurate 'enough'. Which, considering the accuracy of the mapping program and the 'size' of the icon representing my vehicle, will put the reported position within the tolerances of GPS accuracy. This thingy does about the same thing that the 'Delorme' GPS tracking program ('Street Atlas') does, doesn't provide the same statistics, report options though. But, I (and no telling how many others also) put the mess together and made it work. It requires a beacon in the tracked vehicle, a radio receiver to pick up that beacons signal, and a computer to 'mix' the whole thing and give you a picture of where the beacon is. Nothing particularly new about it, and it's done commercially too. I can see applications for model boating, but am just too cheap to try it. Why go to all this trouble when there are commercially available systems to do the same thing? Cuz I'm one of those 'radio freaks', you know?
Oh, the programming was done by one of you 'UK' guys. I'm sorry to say he isn't with us anymore.