Here is the logic for swimming per MCO 3120.11A:
Swim Qualification. Due to water hazards on and near parachute drop zones,
Marine Corps personnel filling valid parachuting billets and/or participating in parachute training on a
permissive basis must be qualified as Water Survival Intermediate (WS-I) at a minimum. All Marine Corps
personnel participating in deliberate water parachute operations will be qualified as WS-I, per reference
(r). Per reference (ah), Marines qualified as Marine Combatant Divers who are assigned to diving duty and meet annual currency requirements per reference (ah
) are exempt from the Marine Corps Water Survival Training Program’s requalification interval.