Does anyone know when the word 'skimmer' first got used to mean 'a low-flying, in-atmosphere hovercraft' in science fiction?
According to Jeff Prucher, Brave New Words: The Oxford Dictionary of Science Fiction (2007), the term in (what appears to be) the relevant sense goes back to at least 1949. Here is the definition of the term in that dictionary, along with the first four cited instances of its use:
skimmer n. a small aircraft designed for low-altitude flight.
1949 W. L. Bade Lost Ulysses [in] Astounding SF (May) 116/1: Tractors shoved the stuff into piles and skimmers mounting gravity engines carried it away.
1957 I. Asimov Profession in Astounding SF (July) 42/2: The skimmer landed at the roof-entry of a hotel.
1965 R. Zelazny ...And Call Me Conrad in Fantasy & SF (Oct.) 11/1: I stood and drew her to her feet as it buzzed in low—a Radson Skimmer: a twenty-foot cockleshell of reflectors and transparency; flat-bottomed, blunt-nosed.
1981 G. R. R. Martin Guardians in Analog SF/Sci. Fact (Oct. 12) 21/2: The Guardians had twenty armed skimmers, and there were another hundrd-odd skimmers and aircars in private hands.