If you add a pool in a back yard somewhere, please add access=private, so data consumers can easily filter them out if they’re looking for actual pools you can go to.
There are already over 5000 swimming pools without a name nor an access tag all over Belgium: overpass turbo
Almost all of them should get an access=private tag. You can fix this relatively quickly, they are easy to recognize from far. Export a few of them to JOSM, select all, shift+e to select the ways that make them, then manually deselect the ones that are actually useful. Or first look at the data, add names to the actual pools (or add an access tag if it is indeed accessible to the public if it really has no name), and run the query again
Note that Shift+E is part of utilsplugin2. Without plugins you can perform a search (Ctrl+F) for type:way selected. (But actually just install utilsplugin2, it’s very useful. )
We should be careful here. I’ve just found a public swimming pool that had no name nor access tag: see Changeset: 161122183 | OpenStreetMap, even if it is in one the best mapped municipality of Belgium
And a swimming pool inside a sport centre can have no name since the name will be on the leisure=sports_centre object, if we follow the wiki:Tag:leisure=swimming_pool - OpenStreetMap Wiki.
Shouldn’t it be access=customers in this case?
Also, yes, this query lets you find improperly mapped “real” swimming pools, like the Ternat swimming pool: Node History: 1990320835 | OpenStreetMap
)