Sephiroth didn’t kill Aerith.
Sephiroth impaled her through a region of the body that doesn’t completely destrouvital organs (at most damage to liver/large intestine).
Her spine is severed at a point where the brain still functions to control the heart and lungs, however it still paralyzes her in the legs and abdomen (which is why she slumps over).
Due to intense pain and swelling of the blood vessels due to being impaled, shefalls unconcious though still alive.
When she is laid to a watery rest she immediately sinks (but dead bodies float) implying body muscle tension is disrupting buoyancy.
Cloud drowned Aerith.
Amusing and disturbing copypasta I found.