So when someone falls overboard the ship could auto deploy floating devices and a trail of floating signal lights/markers. It could also mark the exact GPS spot and alert the crew. After that they could mount a rescue.

The hardest part would be detecting that its a person who's fallen overboard.

You could have people carry devices that trigger the alarm when they get wet and are a certain distance from the deck or use optical sensors - though I think they'd fail more often, especiallyat night.