Wild Waves is probably the largest outdoor water park in Western Washington. They also have amusement park rides, and carnival games, shows, food spots and more!
If you are looking for indoor waterparks in Washington, this is the biggest option for sure. It is the largest indoor water park and resort in the NW area
Slidewaters is a large water park in the Lake Chelan area, so you could plan a getaway around the lake and enjoy the waterpark too!