Books
All good writing is swimming under water and holding your breath.
— F. Scott Fitzgerald · Letter to his daughter Frances Scott Fitzgerald

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
Fitzgerald compares writing to swimming underwater on one breath. Good work means staying submerged in it, without coming up for air. It's a picture of intense, sustained concentration that feels almost like a physical strain on the writer.
Context reviewed