The man, who is in his 50s, must have been out with his son on a leisure boat when the heavy rain and thunderstorm pulled in over the archipelago.
Just before two o'clock on Sunday morning, the son sounded the alarm that the father had been struck by lightning.
The son, who is in his 20s, must have started cardiopulmonary resuscitation himself, before they were both transported ashore with Sjöräddningen's ambulance boat Rescue to Fiskebäck.
- According to my information, both were conscious when they came ashore, says Isac Östman, assistant sea rescue leader at the Sea and Air Rescue Center.
Dad's condition serious
The two were then taken to hospital by ambulance.
The father's condition was serious but stable at 15:30 and the son's injuries were minor, according to Sahlgrenska's press service.
In connection with the storm, Sjöräddningen received several alarms, including groundings.
According to SMHI, more rain is expected in the coming days, and temperatures are also expected to fall.
Very unusual to suffer
It is extremely unusual for a person to be struck by lightning.
The probability has been estimated at around 0.1 per mille chance during an 80-year life.
In Sweden, around five to ten people a year have such bad luck.
However, the vast majority survive.
According to SVT's meteorologist Per Stenborg, only about one in five lightning strikes is fatal.
Dangerous to meet immediately
The most common way an injury occurs is that the lightning strikes an object first, which then spreads and passes the victim.
The most dangerous is to be hit directly by lightning or to be hit by a side charge directly into the head - this can result in fatal output or severe electric shock and severe burns, according to SMHI.
In Sweden, thunderstorms average 5 to 20 days a year, according to SMHI, which can be compared with the Amazon, which can have about 250 thunderstorm days per year.