Fantasy Novel, probably Sword and Sorcery. At one point in the novel, the hero is on a boat that docks in a swampy area, and they are attacked by invisible monsters. They put flour on the deck to see the monster's footprints and kill them, but they also catch an invisible girl that sneaks aboard.
My memory is sketchy except for this part of the book. Probably read in the 70's or 80's