。
Nobody at school can beat her in swimming.
Literal
She [topic-は] swimming [in-では] school [in-では] anyone [to-にも] does-not-lose.
だれにも負けない means 'doesn't lose to anyone' — an emphatic way to say she's the best. 水泳では marks the domain (swimming) and 学校では marks the scope (at school). The double では narrows the claim to a specific area within a specific context.