You’re allowed to wear a speedo both in the water and on the pool deck. If you feel that to lounge about on the pool deck in a speedo is not bekovidik then feel free to wear a towel or robe. But in regards to halacha, it is perfectly acceptable to swim in a speedo and walk around the pool deck as well. You’re allowed to immerse in a mikva with other men present, so we don’t forbid nudity among men, but we do encourage a certain level of self-respect towards one’s body. That has much to do with the environment one is in. It is perfectly appropriate at the pool to wear a speedo.
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