A mother found her daughter collapsed in the bath after her heart suddenly stopped, an inquest heard.
Fit and healthy Amber Rummels, 18, was found under the water with the taps still running at her home in Cardiff.
Anita Lewis pulled her out of the bath and gave her CPR before paramedics managed to start her heart, but she died four days later in hospital on 5 December.
Coroner Graeme Hughes recorded a narrative conclusion in Pontypridd.
A post-mortem examination found Amber died from hypoxic brain injury and a rare complication of pneumonia from nearly drowning due to sudden arrhythmic death syndrome (Sads).
Sads is when the heart stops beating unexpectedly due to an abnormal rhythm, and generally happens to younger people who do not know they have a heart problem.
In a statement, Ms Lewis said she woke in the night to find the house with no electricity.
She said: “Water was coming through the light fitting in the hallway.
“I could then hear a dripping sound of water coming from the bathroom.”
The inquest heard she grabbed a pen and managed to unlock the bathroom door.
She said: “I looked into the bath and I saw the water was running from both taps. Amber was lying in the foetal position.
“I grabbed Amber out of the bath under her arms and I put her on the floor.”
Delivering his conclusion, Mr Hughes said it was “possible but unlikely” an electric facial exfoliator in the bath contributed to Amber’s arrhythmia.
The teenager was studying hospitality at Cardiff and Vale College.
Her father Steven Rummels said: “Amber was a strong character. She was fun and caring and we will all miss her dearly.”