You need to get to the area and start looking for rescue flares on the water. If you are playing with map contacts on, you will see blue squares on the map. Just head towards them and start looking for the smoke. You must be surfaced to pick them up and you have to get really close and stop.
You lock on them with the TBT or periscope using the "L" key and the little life preserver will be available for you to press on the HUD.
Now they won't stay afloat forever so time is of the essence.
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