The Venetian Pool is excellent!
This historic site, tucked inside the neighborhood of Coral Gables in Miami, was originally a rock quarry site that was turned into a really large Venetian-styled pool area. The really large pool also has a waterfall and cave. There's a sort of large sandbox for sunbathing, and outdoor showers for rinsing off.
Since it is filled with spring water, the waters are much colder than the ocean water here. Also, when I went, there were a lot of kids in the area. But if you enjoy swimming at all, I'd rank this as one of the coolest pools you'll ever be in. It's definitely a must-see.