A person who takes a bath or swims in water for pleasure or exercise.
From Old English 'bæð' (bath) plus the agent suffix '-er' (one who does something). The word evolved in Middle English as bathing became a common activity, and '-er' was regularly applied to create occupational or descriptive nouns.
Interestingly, 'bather' wasn't commonly used until the Victorian era when seaside bathing became fashionable; before that, people mostly said 'one who bathes' since recreational swimming was rare for most of history.
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