The idea isn't new for full sized boats, but for R/C sized boats it certainly would be. Biggest problem is miniaturizing the whole thing, not to mention financing it. I think that 100 - 150 Pound range is -VERY- optimistic! The accuracy of such a GPS based navigation system is 'workable' for 'full sized' things. That accuracy is almost 5 times(?) the size of a typical R/C boat. That might get the boat back to a bank, but I don't think I'd count on it being near your feet, on that bank (unless you plan to do a lot of walking, sort of). Now if you can 'scale' the accuracy up as much as the R/C boat is 'scaled' down... Oh have I got a deal for you!!
- 'Doc