Coming from the UK I doubt that he would even been are that sea snakes were in the area, he may of heard of them but they would of been the last thing on his mind.
The UK only has 2 types of snakes, One is the grass snake which is not poisonous and the Adder which is a member of the Viper family and it can't bite through ya jeans but it is poisonous, with most people it will make em sick and a bit sore and it could kill people with a heart condition or very small children and they grow up to 3 ft long, but I have never seen one or known of anyone to have been bitten by one.