Translation:I do not swim.
泳ぎません is "I will not swim" (I don't intend to swim in the immediate future) or "I do not swim" (as a habitual action, it is something I do not do).
You might be able to swim, but you just don't want to. Or you might be unable you swim. It does not make that clear.
If you want to specify that are not ABLE to swim, there is a different way to say it.