The body of what is thought to be a minke whale has washed into the Bristol Channel in Somerset.
Derek Jones and his 12-year-old son Sean, from Bristol, were on sea scooters on the River Parrett when they spotted the 33ft (10m)-long mammal.
It was thought to still be alive and the pair unsuccessfully tried to tow it back out to sea.
The body of the animal was last seen floating into the Bristol Channel at Steart Island near Burnham-on-Sea.
Steve O'Connor from the British Divers Marine Life Rescue said when the report came in he had 15 marine medics ready to go out and rescue the whale.
He added: "There are a number of reasons why it became stranded.
"It could have had a navigational error that made it take the wrong turn or it could have been under stress.
"We will not know the exact cause of death unless a vet examines it. The body may come aground again elsewhere but the majority of times it will just float away.
"It was definitely fully grown and fairly big. They have been spotted in the Bristol Channel before but very rarely.
"Around the UK we will deal with about 50 strandings a year of dolphins and whales but it is unusual for it to be so far up the estuary."