Shocked Dan Muldowney was left in agony, bleeding and feeling dizzy for ten days after the attack.
The 4ft creature sank its fangs into his bare shin while he was bodyboarding on holiday with his family – leaving a double puncture wound.
It also bit him on the thigh but failed to penetrate his knee-length wetsuit.
Dan, 27, who believes he was the victim of a tropical species normally found in the Pacific and Indian Oceans, said: “I felt something like a clump of seaweed against my leg, then a bang.
“When I looked in front of me I saw a snake with a black head shaped like a cobra – I clearly remember seeing its wide neck. No sooner had I seen it than it was gone.”
The attack at the popular resort of Newquay came just weeks after a fisherman saw a sea snake swimming at Whitsands in south-east Cornwall.
Dan, a surveyor, was enjoying a break with girlfriend Kenize Akhtar and their four children when he was bitten.
At the time he was in chest-deep surf trying out a new wetsuit he had just bought.
To begin with he thought he had hit his leg on a sharp rock and seen a huge eel.
But his wound began bleeding heavily and within ten minutes he was out of breath.
The following day blood started oozing from his mouth and he felt so unwell he was on the verge of collapse.
Then lumps like bubbles appeared on his leg and his fingertips began to tingle.
He consulted a GP, who was baffled by his symptoms, before a second doctor sent him to hospital where he was given allergy tablets.
By the time he returned home to Oldham, Gtr Manchester, he had headaches and twitching muscles.
Dan said: “My nerves were an absolute mess. I was irritable and unable to cope. Some days I just stayed in bed.”
Hospital tests failed to show anything conclusive but 12 days after his ordeal Dan started to recover.
After doing research he is sure he was attacked by a yellow-bellied sea snake. The creature’s venom, used to kill fish, is ten times more lethal than an Egyptian cobra’s.
Dan dismissed suggestions that he might have been bitten by an adder which had fallen off cliffs into the water.
He said: “There is a report of a Chinese man attacked by a sea snake who took 77 hours to die and his symptoms were the same as mine. I just want to warn other people.”
Experts said British coastal waters were too cold for sea snakes to occur naturally but that Dan might have been bitten by one which was DUMPED.
A Marine Conservation Society spokeswoman said: “If someone had kept a sea snake and let it go, potentially it could survive.”