Was the spruce goose the first flying boat?

No. In 1913, Curtiss introduced the first commercial flying boat, the Model E. There were regular flights of air boats in the UK starting about 1920. The Spruce Goose didn't come along until the 1940s.

The Spruce Goose was the largest plane to fly. Before the Spruce Goose, pilots adjusted the control surfaces of a plane with their own muscles, but the Spruce Goose was so large that power controls were required. Howard Hughes decided to park the plane without offering any explanation to anyone, apparently being dissatisfied with the performance of the control system - and of course, now we will never know.