A 15-year-old boy has died after falling off rocks near Llangennith, Gower.
His 21-year-old brother dived into the sea in an attempt to save his brother but the pair had to be rescued.
They had been fishing with two other friends at Blue Pool corner on Wednesday evening.
The pair were airlifted to Morriston Hospital, Swansea, where the 15-year-old was pronounced dead.
His older brother remains in hospital, said South Wales Police.
Det Insp David Hough said: "This was a very tragic incident and our thoughts are with the family at this sad time."
The alarm was raised at 20:00 BST when a third member of the group ran to a nearby beach and used a walker's mobile phone.
A fourth person kept watch over the brothers until help arrived.
Swansea Coastguard, Rhossili Coastguard, the Loughor Inshore Rescue Lifeboat, Burry Port RNLI inshore lifeboat and the Tenby RNLI and a rescue helicopter all attended.
One brother was picked up by Burry Port lifeboat and the other was winched from the water by helicopter.
Will Parfitt, watch manager at Swansea Coastguard said: "The coastguard recommends that anglers who are fishing on their own close to the water's edge and from an unstable platform, such as rocks, should wear a lifejacket or buoyancy aid."
