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 daylight, the range is less than ~200 meters, the chopper is almost right above them, the depth of the divers is shallower than ~10 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.