Lake Union in Seattle is a freshwater lake that is along the Lake Washington Ship Canal. It is a very active body of water with a large number of recreation boaters, kayakers, and paddle boarders, sightseeing ships, and, yes, seaplanes. In the image above, you see a Kenmore Air de Havilland DHC-3 Otter lifting off from Lake Union on a sunny June morning. Lake Union is also the location of floating homes (like the one seen in the movie, Sleepless in Seattle).
This photo was taken from Gas Works Park on the north side of the lake. On the south side of the lake (South Lake Union) is the Museum of History & Industry (MOHAI) housed in the former Naval Reserve Armory.