Darker Than Amber by John D. MacDonald (1966-05-03)

Travis McGee #7 [From Wikipedia] Darker than Amber (1966) is the seventh novel in the Travis McGee series by John D. MacDonald. The plot begins with McGee and his close friend Meyer are fishing underneath a bridge and a young woman, bound and weighted, is thrown over the bridge. It was also adapted into a 1970 film of the same name.