My father and I fill our 20k gallon inground every year with water from a creek about 250 yards down the road. We pump out the water left in over the winter, powerwash the liner and get everything ready to go about a week or so before we fill it. We use a trash pump and 2-1/2" firehose, it takes about 12 hours to fill. No filters except for a strainer basket on the pump-end.
Using the creek water is MUCH MUCH faster than trying to bring the stagnate water around, and much cheaper as far as chemicals are concerned. The crap drops out in a day or 2 of shocking it, a D/E filter, and careful application of chemistry takes care of the rest, its crystal clear and swimable in a week.
As far as legality is concerned, we're out in the sticks. The local volunteer fire departments are much more eager to lend you a dozen lengths of hose than they are to use their limited resources to fill your pool. Likewise, one of our neighbors complained about where we routed the hose one year (we had asked his wife if she minded about 6 hours before), and called the sheriff. The sheriff drove up, saw what we were doing, looked our setup over, nodded, and drove off without so much as a word. So long as you aren't being an idiot, or going out of your way to inconvenience anybody, I doubt anyone will bother you about filling a pool using a river, creek, or large pond.
