If you like to swim then there is one pool that is head and shoulders and torso above the rest: Colman pool in West Seattle. A Seattle landmark, this olympic sized pool has it all: it's uncrowded, outdoors, heated, saltwater, adjacent to a beach and has views of the Olympics. They even have 3 hour swim sessions so you'll have time to enjoy the 1m and 3m diving boards and a giant tube slide!
Why isn't this pool jam packed? Well, you have to walk about 1/4 mile from your car, through lovely Lincoln park trails or along the paved beach path to get there. So if you are one of the few who is undaunted by the grueling hike, come and enjoy the best swimming pool around.
Half an hour south of Seattle you'll find Wild Waves. It's as close as Washington and Oregon gets to a big water park. Although it's slightly over priced and definetly can be packed in the summer it's a great little day trip if you've got kids and are visiting Seattle in the summer.