... in recent years our local swimming pool has been open during the 9 Days, though restricted to adults.
The 9 Days, which falls during the heat of the summer (in the northern hemisphere) is a period of mourning in the Jewish Calendar. We mourn the destruction of both the First and Second Holy Temples. The period of annual mourning is actually Three Weeks, from the daytime Fast of the 17th of Tamuz until the 25 hour Fast of the 9th of Av.
Just a few decades ago the idea of swimming, even for health reasons, during the 9 Days would be unthinkable for Torah Observant Jews, but nowadays it's the norm for many of us who regularly swim or do water exercise. Until a few years ago, our local Shiloh swimming pool was closed during that time of the year, but then one of the neighbors explained to the rabbi that, at least for adults, it wasn't a matter of fun and games. He then agreed that the pool could open, but adults only. So now we have special hours each day, separate male/female as is our custom.
Not everyone who normally swims in the pool takes advantage of the new psak/decision, but there are many who are very grateful for the chance to exercise in the water daily. Yes, I'm one of them.