Mauritius, page-6

maybe 3km from shore is to do with the bathymetry. Mauritius is a volcanic island and I suspect the seafloor would plummet from he "continental shelf" fairly rapidly. I was trying to find some marine charts but not really successful - the seafloor i appears to drop fairly quacking on google maps maybe 5-10km from shore but it's hard to determine as it's not proper bathymetric data. I went there in 1984 and I think they had breakers on the horizon. So maybe the 3km is a physical constraint.

This you would assume would be quite significant issue/ constraint on places in the pacific where the shelf is narrow and the continental slope is quite steep.