The signature appears on a boat.
A related drawing in Edinburgh, National Gallery of Scotland; Ertz thinks that this is not a study for the painting but a later copy after it.
Evidently there is a copy that was sold in London (Sotheby's November 25, 1970, #103; panel, 55 x 94) which is by an anonymous follower again according to Ertz; Weid calls it a work by Jan the Younger.
quite good copy on canvas (52.5 x 92.5) Wizowitz, Slot, inv. #1771/1591 (photo RKD)