It's your definition of 'wastewater' that matters.  In most modern conventional power plants, the water used to create steam is reused - up to 80% of it - by condensing the steam.  The rest evaporates or is drained, and that is made up from a raw water source.  That water is demineralized and degassed, then added to the steam condensate.  Steam condensate has chemicals added to it for anti-corrosion, pipe protection, and any residual mineral and oxygen removal.  So no, you wouldn't want to swim in the water used to make steam; the chemicals would really mess you up.  But the water used to cool plant process systems is often drawn from and returned to a natural source, a lake or river.  That water is the same as the natural water source, only warmer.  Depending on the size of the lake, swimming in it would be okay, although I wouldn't get too close to the plant site.  A river would be fine as any contaminants will be quickly diluted.