The ornamental design for a drybag with zipper, as shown and described.
FIG. 1 of the drawings is a front elevational view of the dry bag with zipper in a flattened configuration, showing, in particular, the ornamental design thereof;
FIG. 2 of the drawings is a back elevational view in a flattened configuration thereof;
FIG. 3 of the drawings is a first side elevational view thereof; and,
FIG. 4 of the drawings is a second side elevational view thereof.
The broken lines showing stitching and zipper tread form no part of the claimed design.