The ornamental design for a seadrogue for a boat, as shown and described.
FIG. 1 is a bottom plan view of a sea drogue for a boat which forms my new design, the broken lines in this and other figures being understood to represent stitch lines;
FIG. 2 is a right side elevational view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a left side elevational view thereof; and,