On the 2G4 radios that I got instructions with, they usually claim 1Km for air use, 500m for surface. This is probably true enough for most makes of "full range" sets. "Park Flyer" radios have either less transmitter power or less sensitive receivers, or both. For surface use, it depends on the size of the water what range is needed, but even half of a Park Flyers claimed range might be enough. Modern digital gear works amazingly reliably until it gets to its range limit, then it stops very suddenly. Also how much care is taken placing the last inch of the antenna wire - thats the bit that does the business.