The ornamental design for a support line clamp for boat accessories, as shown and described.
FIG. 1 is a front view of a support line clamp for boat accessories, showing my new design;
FIG. 2 is a right side thereof, the left side view being a mirror image
FIG. 3 is a top view thereof, this view having one of a pair of threaded clamping nuts removed for clarity of the illustration, the bottom view being a mirror image thereof; and,
FIG. 4 is a right side view of FIG. 3 and showing an interior beveled face of each of the threaded clamping nuts.