(away from books, so take with grain of salt removed from purified water)
I've had ideas along the lines of separate salt from water for sale, and generally separating impurities to make, say, really good glassmaking sand. Good if you happen to live on an island with sandy beaches in the middle of the Mediterranean.
Now if you want a spring... Isn't the 'create spring' effect a carryover from previous editions? Sorta based on 'Moses strikes the rock and summons a natural spring from the ground', and the spring was as self-sustaining as a natural spring after that, which in this edition would be more complicated. Depending how you interpret it: if it is just water coming up from the ground as if a spring, then the total water you get should be about one individual of water. If it really is a natural spring, just 'summoned' with Creo, then after some time (days, seasons, depends on game?) it can do things like dry up, become contaminated, suck water out from surrounding area, etc etc. For a fake spring, also consider Rego to teleport the water from a river - just have it come out like a spring or a well to keep locals from getting superstitious. For a decorative fountain or to cleanse 'foul airs' or anything where the water doesn't need to be permanent, perhaps a Muto effect to turn air to water
Once you have the water, various ways to purify it. I would rule that Perdo/Muto/Rego can destroy/turn into bubble/pull aside into a collection chamber, one impurity. A In/Re Te(Aq) spell, "Maxwell's Daemon", that makes a filter that lets water through, while blocking salt. So one spell can remove salt, but won't do anything about the algae, mud, or jellyfish poison. Creo would fix everything, 'turn foul water into sweet water' with one spell. Hmm. Doesn't totally salt-free water (nowadays known as distilled or de-ionized) taste funny unless you add a teeny bit of salt back in? That would give the Creo version another edge - good water not ill-tasting but salt free water
For actually implementing the purification, you have two options. A, purify the whole body of water. See other people's posts. B, do a magical modern-day-pool filter. Route the water through a magic box or magic rain barrel enchanted with a purification spell. If you have a salt lake, routing through the filter and back into the lake will eventually desalinate the whole thing. Or just filter the stuff as it leaves the lake to go into your plumbing system.
Random weirder idea, going back to Athena, Neptune, and the olive tree. Could you use Herbam to make a tree that desalinates water as part of its normal life? Call it an 'olive tree of virtue'.