If you want to swim in the mornings the Green Lake Aqua Ducks is a good option. Can't say I'm a big fan of that pool, though. There's a masters group that swims at Queen Anne a couple evenings a week and on Sunday morning. A good after work option is Husky Masters at UW, but right now the pool is undergoing repairs, so their schedule is a little funky. Most of the Seattle pools also have masters workouts at various times. These can be fine if there's a decent group, but don't expect much for coaching. The workouts are usually written by one of the lifeguards on a white board and that's about it for organization.
Once Colman opens in three weeks that opens up more opportunities and it's one of the most scenic pools I've ever swum in! You can find schedule info for all the city pools here: