which (until this post) was used as a phrase in only one Google-findable source, turns out to be expressly regulated (since 1956) by the New Orleans Code of Ordinances:
Sec. 170-60. Swimming, fishing or scuba diving prohibited in skiing or armed scuba diving areas.
(a) It shall be unlawful to swim or bathe or fish or scuba dive in the following areas of Lake Pontchartrain wherein water skiing is practiced:
That portion of the lake from the east breakwater of Lake Pontchartrain Beach to Franklin Avenue, extending from the shoreline into the lake a distance of 200 yards.
That portion of the lake from the west breakwater of Lake Pontchartrain Beach to London Avenue Canal and extending from the shoreline into the lake a distance of 200 yards.
That portion of the lake between the extension of Canal Boulevard and the extension of the Orleans Canal for a distance of 200 yards from the shoreline.
(b) It shall be unlawful to swim or bathe or fish at any time in the following areas of Lake Pontchartrain wherein armed scuba diving shall be confined:
The first 50 yards from the north shoreline of the Municipal Yacht Harbor breakwater, extending from the Orleans-Jefferson Parish line to 50 feet from the east end of such breakwater, to be measured westward 50 feet from such east end.
The first 50 yards along the entire length of the east side of the New Orleans Airport seawall.
(c) Scuba diving without weapons, fishing, swimming and bathing shall be lawful in all other areas of the lake not prohibited by this section and by section 170-59, which shall remain in full force and effect.
Yet despite this express authorization, the New Orleans armed scuba diving scene is apparently very low profile (at least under that name).
Thanks to Stephanie Plotin for the pointer.