Note that even though the choppers ammunition cannot harm submerged divers, this behaviour breaks stealth mission.

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Choppers can detect submerged divers in conditions that a human pilot will not be able to detect them.
Under no circumstances should a choppers be able to detect a diver during the night, and/or a diver in deep dive.

Choppers should detect submerged divers ONLY under the following circumstances:
It's broad daylight, the range is less than about two hundred meters, the chopper is almost right above them, the depth of the divers is shallower than about ten meters. If it's night a chopper can detect divers only if they are on the water surface and the pilot is using NVGs or using an illumination device.