Richard Bull, 32, is believed to have plugged the charger into an extension cord from the hallway and then rested it on his chest while using the phone.
He suffered severe burns on this chest, arm and hand when part of it touched the water.
Wife Tanya found him when she returned home and first thought he had been attacked because his injuries were so bad.
She called 999 but paramedics said he was already dead by the time they got there.
Recording a verdict of accidental death, Dr Sean Cummings said: “These seem like innocuous devices but can be as dangerous as a hairdryer in a bathroom.
“They should attach warnings. I intend to write a report later to the makers of the phone.”