。
She's proud of her swimming.
Literal
She [topic-は] swimming [subject-が] pride is.
自慢だ (じまんだ) means 'is a source of pride' or 'is something to boast about.' 水泳が自慢だ uses the XがY pattern: 'swimming is her pride.' 自慢 can be positive (justifiable pride) or slightly negative (boasting). Context determines the reading.