Is swimming in underwear acceptable instead of costume in swimming

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Should you wear underwear under swim trunks?

Are you supposed to wear underwear under men's swim trunks? If so, which type? - Quora

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Are people allowed to swim with clothes on in an Australian swimming pool? - Quora

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Is it bad to wear too tight shorts when swimming in a public pool? - Quora

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Do men swim in their underwear, and why? - Quora