Thirty-two-year-old Richard was discovered by his wife and had such severe burns to his chest, arm and hand that she thought he had been attacked.
He is thought to have used an extension cord running into the bathroom from the hallway then rested the phone on his chest.
When it touched the water, the phone electrocuted him, killing him immediately.
The coroner recorded a verdict of accidental death, saying: “These (iPhones) 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.”
His mother said: “I worry so many people, and especially teenagers who can’t be separated from their phones, don’t know how dangerous it is.”
The incident happened in Ealing, West London; and was reported in the SUN.