The need for water is a huge issue to some wildlife as super warm water doesn't exactly help fish if they are acclimatized to something a couple dozen degrees colder. We are working on it . Unfortunately the phase cycle of water is our best way to tap the power at the moment. Not to give people the wrong impression, something like a CANDU has 2 completely isolated water loops (the core and the generator), the water from a river is only used to condense the steam and is never part of either loop.
Our riverine fish (Tennessee) are more tolerant of warmer water; our biggest problems are loss of riverine habitat (especially breeding habitat) from the reservoirs (both for nuke cooling and for hydro) and cold, oxygen-poor water from hydro generation. The former has been partially mitigated by some judicious creation of levees and artificial gravel bars, and by artificially rearing species that don't breed well under reservoir conditions. The latter has been partially mitigated by mechanically oxygenating the cold water, by mixing in some surface water to warm the spill, and by stocking cold water species like striped bass and even trout. In theory the warmed water from the nuclear plant's cooling loops could be used to warm the cooled water from deep water hydro generation, but in practice it hasn't yet been tried. Since we also use the reservoirs for hydro generation we get a nice boost from the nuclear and the added heat from the nuclear plant cooling at least partially offsets the cold hydro outflow. The added heat can be a stressor in the summer, but most of the flow-through reservoir stays about the same temperature and winter kill-off (granted, mostly among introduced shad species) are reduced. This is actually a quasi-government agency BTW, the Tennessee Valley Authority, which is both an example of government's scope creep and an example of government doing a pretty decent job.
There are worse problems to have; some of the Georgia nuclear plants have had to shut down some summers because in dog days the water gets too hot to condense the steam and cool the plant!