When did sailors have to know how to swim?

When did swimming ability become a required skill on European vessels?

There was no early uniform requirement; compulsory swimming was still being debated during both world wars.

The question follows an episode in which Kowshing's crew had to abandon ship under fire and swim toward rescue. Compulsory swimming proficiency on European vessels was still being debated during the First World War and even the Second. The answer warns against projecting modern safety expectations backward onto nineteenth-century sailors. It also places basic survival training beside the much faster development of ships and weapons.