At that time, near the coast, some people were practicing rescuing people who have drowned using a drone.
The people who new the boys had drowned flew the drone to the open sea.
The drone dropped a device to rescue the people who had drowned right next to them.
I don't understand why 溺れた is used in these sentences, rather than 溺れている, or even 溺れる. It seems to me that at the time of rescue the people are either drowning now or will drown rather than having already drowned.
I'm not sure if I'm having a problem with this specific verb or if it indicates a bigger hole in my understanding of aspect in relative clauses. I tried reading this, this and this but I have to admit to being completely mystified.