Cells whose entire footprint is open water (ocean, large lake) carry no population/audience but can still be generated by a naive grid fill over a bounding box or coastal buffer.
▪ Rendered from the real conversion/geometry code.
Detection
Cell polygon land-fraction approximately 0 against a land/water mask.
Mitigation
Mask against a land polygon before finalizing a cell set
Exclude or down-weight zero-land cells in audience/delivery counts