Blazing confidence and wrapped in a leather bomber jacket, Paul Watkins was Mademoiselle magazine's March ''Hunk of the Month.'' Shirtless with a 16-pound salmon on his chest, he was the sex god in a British Levi's ad. When he isn't the raw material for other people's copy, the 26-year-old Watkins writes his own. The author of two highly praised novels (Night Over Day Over Night; Calm at Sunset, Calm at Dawn), Watkins has set new standards in the burgeoning field of literary pinups with macho pastimes; for his new novel, In the Blue Light of African Dreams, just published by Houghton Mifflin, he learned to fly a biplane and spent months in the Sahara. ''It's very easy to tell when a writer hasn't been someplace,'' he says. ''You never have the coin between your teeth, so to speak just the wooden nickel.''