Known by many different names, spraygrounds, wading pools, & splash parks are a great way to beat the heat without worrying about your kid drowning! Here is the link to Tacoma Parks & Rec. where you can find one near you. I'm not sure if Seattle has actual splash parks, but their P&R link shows a zillion wading pools.
http://www.metroparkstacoma.org/page.php?id=69
http://www.seattle.gov/parks/aquatics/index.htm
Auburn Rotary Spray Park at Les Gove Park - Sprayground is new and fun. Turn in and park near the library. Every time I've gone it has been very crowded to the point of turning my toddler to play in the dirt (so glad we went all that way to play in the dirt).
Jefferson Park (Tacoma) - Wonderful sprayground, playground, and picnic areas. Lots of shade. Parking close. Doesn't feel crowded even with a lot of people. Look for the ice cream truck that circles by often.
Pioneer Park Wading Pool (Puyallup) - Check for hours as they vary. Season is short and doesn't start until mid-July. Nice place to go with a group. Library story times start around 10:00 and 11:00 a.m. Wading pool opens at noon unless there is a summer concert, then 1:30 p.m.