what baby should wear when swimming?

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i just bought an inflatable pool for my 1yr old, im also planning on getting him a reusable swimming diaper. but if i were to buy it, can he wear it alone wout having to wear a regular, disposable diaper under it? or does he have to wear both or one of them under a swimming short? or i dont have to buy any swimming diapers and he can just wear his regular diaper in a swimming short?