Seattle's famous Crystal Pool natatorium was located at Second Avenue and Lenora, southwest corner, in the Belltown district of town. (It was originally called the Seattle Natatorium. The other major marine natatorium at the time was located in Victoria, British Columbia.) It was a giant bathhouse that pumped in heated saltwater straight from Elliott Bay (Puget Sound), and existed as a public swimming pool from 1915 (or 1916, depending upon the source) until it was closed down in the late 1930s.
It had been designed by B. Marcus Priteca, known as a leading designer of movie film theaters, including Seattle's Coliseum movie theater that still exists to this day, after major renovations financed by Microsoft co-founder Paul Allen. 
By 1920 it was controlled by the Metropolitan Building Company, at a time when Seattle boxing had "hit a snag." (Oct. 26 Tacoma News Tribune)
The inaugural boxing show was conducted Nov. 5, 1917, when Austin and Salt leased the south end of the building, drained the pool, and installed a ring platform and bleachers—seating 2,000—into and around the empty pool. Oct. 25 Seattle Daily Times. "The Pool" thereafter offered boxing cards in the Fall and Winter seasons. (Due to the lack of in-door air-conditioning enjoyed today, among other factors—including baseball—boxing was considered an in-door sport in those days.) The Pool was also a roller-skating venue in the winter, among other sports, as well as swimming place during the spring and summer seasons.
By early 1918 Austin and Salt were staging weekly shows at the Pool, drawing $1,500 to $2,000 houses each time, and making boxing the most popular sport of the season. 
Starting December 9, 1935, the Crystal Pool hosted the first annual Northwest Golden Gloves tournament (which featured future heavyweight Wild Bill Boyd of the U.S.S. Idaho knocking out Chuck Kinzel of the Washington Athletic Club).
According to Larry Kreisman, one-time Seattle Landmarks Preservation Board historian: "The Crystal became one of the most popular recreation spots in the city. The swimming pool was spanned by enormous arched steel trusses supporting a glass roof. Surrounding the pool were tiers of seats for as many as 1,500 spectators. Salt water for the pool was piped from Elliott Bay and heated, filtered and chlorinated. The principal exterior change has been replacement of the entrance pavilion. Modifications have been made to the interior for an assembly space but the building still has the pool's tile edging and its bleachers. A developer plans to demolish the building and build a condominium tower. The church ownership places the building outside city Landmarks Board jurisdiction.... This is a unique building type; there is no other in Seattle. It is a part of the collective memory of the city, how it functioned, how we lived, how we worked and how we recreated."
The Bethel Temple Church bought the building and opened in 1944.
The old Crystal Pool building was completely remodeled in 2005 into mix-use condominiums, maintaining parts of its old eastern and northern facades (shown in the photo above), and is currently known as the "Cristalla." A plaque on the northern facade mentions the building's history, but seems to imply, incorrectly, that it was used as a boxing venue only after it had stopped being used for public swimming.
A chapter in the 2011 book A Dream, A Buck, An Era is devoted to this old-time boxing venue.
1927 photo at top of this page is courtesy of the Museum of History & Industry (MOHAI), Seattle
- Photo of exterior (circa 1915); another exterior photo (date unknown)
- Photo of exterior, as Bethel Temple
- Photo of interior, when used for swimming
- Boxing Advert
- Image of 2008 Cristalla Building