I had my Mavic Mini ditch in the Sea today too. Very good conditions, the wind was barely noticeable, UAV was responding well and everything seemed good. On the previous flight I'd run the battery down to 15% (Landing slightly delayed by people walking along the promenade). Visual inspection of the props was genuinely performed, props were at 29 flights / 2.75hrs
I pressed RTH, saw a nice picture and stopped for a moment. Pressed RTH again, got an overload warning and the thing just descended into the sea. I tried to command it to fly up when I realised it was dropping altitude / about the hit the sea, but got no response. It dripped into the sea, then jumped back into the air for a few seconds before dropping down again.
Home point was defined correctly, RTH was 15m from the sea defense, which was about 20m above the sea.
Drone hasn't been recovered yet... I doubt it will be
For those who have had success with replacement from DJI, how did you initiate a support ticket?