Adam Bates, a member of Shoreham Rowing Club, was around five metres from the shore on Kingston Beach when a seal popped up next to his single scull and clambered up onto the bow.
He said: “It was wonderful, I’d never seen anything quite like it.
“It was amazing.”
Adam, who lives in Brighton, admitted he was a bit nervous about the seal rocking the boat.
He said: “Those boats are really unstable – I was a bit worried about falling in! They are a bit tricky to get out of.
“But it didn’t tip the boat very much at all.
“It put its weight firmly down and kept it nicely balanced.
“It was a lovely moment.”
Local wildlife photographer Sean Stones was on the beach taking photos of the seal when he captured the moment it climbed aboard.
He said: “I was lucky enough to spot the seal on the drive home from work and even luckier was that I had the camera with me.
“I watched it for a while until it headed into the water, just as the rowing club were launching.
“The seal followed Adam out into the harbour and decided to climb aboard the one man scull.”
The seal has been spotted several times in the Shoreham area over the last two or three weeks.
Sean has posted several photos of it on his Facebook page WYLD: Photography.
To see more of his images, search for @seanstonesnaturephotography on Facebook.