The Grumman Goose seaplane actually exists and is drawn very realistically, but its actual range is even shorter - just 557 nautical miles.
The Grumman Goose's floats are actually retractable and form the wing tips, yet, for some reason, Rip Riley never retracts them, even long after the start, when cruising. Maybe the retraction mechanism is supposed to be broken; it is never explained. In any event, the increased air drag of the lowered floats would reduce the range still further.