How to maintain a pool without eventually having to replace water?
You don't say much about the type of pool (in-ground,above ground, lined, concrete, etc) or the environment (is freezing an issue) but our in-ground vinyl lined pool in the southern United States uses a salt water chlorinator and the water has not been replaced (aside form evaporative losses) in the 5 years since the pool was installed.  It doesn't get much safer (just add salt) or easier.  We get the water tested at the pool supply store and have to add a bit of this or that from time to time, but overall it is hassle-free.