Billy Obeney shared the sighting on social media after a morning walk at Holywell, in Eastbourne, describing a “very large fin moving gracefully across the sea about 20 metres off the edge of the beach.”
Before spotting the fin, he had seen a seal pup beneath Holywell Café.
Suggestions ranged from dolphins to basking sharks, with Obeney noting: “I think the seal pup was scared off by this animal.”
Gary Tester said: “Definitely a dolphin fin.
“Awesome sight to see so close together.”
David Papworth added: “Seals and penguins are sharks’ main source of food – that’s why seals leave quickly onto the beach.”
However, Sarah Ward, marine conservation officer, said the fin likely belonged to a grey seal.
Ms Ward said: “The head of a resting Grey Seal can be mistaken for a shark’s dorsal fin.
“This is because when seals are resting at sea, they bob around in the water with only their head or back above the surface.
“This is called “bottling”.”