If you have active folliculitis/Sweet's syndrome, it is probably best not to go swimming. This is because the skin lesions may be irritated by contact with the chemicals in the water, if you are using a swimming pool. On the other hand, swimming in fresh water might run a risk of making the infection worse through contact with the bacteria and other organisms living in the water. Generally speaking, it is best to avoid swimming whenever you have an active skin rash or skin infection.
With Sweet's syndrome, once you have been taking the medications for a while and the worst of the inflammation is over, you should be able to resume activities, including swimming. However, I would defer to the doctor
who is treating the Sweet's syndrome. Although Sweet's syndrome is sometimes an isolated finding, it is often associated with underlying medical conditions. Resuming activities like swimming might also be dependent on whether you have another underlying medical condition and whether that condition is also under control.
Please be sure to touch base with your primary care doctor
or the doctor treating your Sweet's syndrome, as they know your situation best!