Why couldn't Scout go to Barker's Eddy, the swimming creek, with Jem and Dill?

Scout wasn't allowed to go swimming with the boys at Barker's Eddy because they were going to swim naked.

August was on the brink of September. Dill would be leaving for Meridian tomorrow; today he was off with Jem at Barker’s Eddy. Jem had discovered with angry amazement that nobody had ever bothered to teach Dill how to swim, a skill Jem considered necessary as walking. They had spent two afternoons at the creek, they said they were going in naked and I couldn’t come, so I divided the lonely hours between Calpurnia and Miss Maudie.