An unexpected noise source has been found in mag-netic- and sometimes electric-field data recorded on the bottom of lakes in the Archean Slave craton (northwestern Canada) during warm seasons. The noise is the result of instrument motion and in some instances direct induction by wind-driven surface gravity waves when the lakes are not ice covered. The noise can be reduced or eliminated by prefiltering the data with an adaptive correlation noise-canceling filter using instrument tilt records prior to estimation of magnetotelluric (MT) response functions. Similar effects are to be expected in other shallow-water environments, and the adaptive correlation canceler is a suitable method to preprocess MT data to reduce motion-related noise in the magnetic field. This underscores the importance of ancillary tilt measurements in shallow-water MT surveys. In coastal or lake-bottom surveys, special efforts to reduce hydrodynamic effects on the instrument should also be pursued.