How were these GPS points obtained?
An overview of the marine outfalls was provided in the document "Water services and the Cape Town urban water cycle" (2018) by the City of Cape Town. Coordinates were reviewed on Open Street Maps.

Are there any known data gaps?
The locations are approximate locations; the exact coordinates should be checked, perhaps by reviewing permits or the water and sanitation department, or perhaps by going to the relevant beaches and identifying the outfalls during low tide.