Serrasalmus nattereri, or the red bellied piranha, is "supposed" to only be able to live in warm water and only grow to 11" long. I did a brief study in 1985 that concluded that the normal habitat, the southern Amazon basin, freezes solid 2 months out of the year with no adverse effects on the local piranha population. This significantly increases the breeding ground and sustainable environments for them.
There is also video and pictorial evidence of 20-25" long specimens and historical record from the Theodore Roosevelt expedition of Piranha biting through 1.5" heat treated steel meat hooks.
Maybe YOU want to go swimming with those poor scared little boogers but I won't go near water with Serrasalmus nattereri.
Heres the problem with that though.The waters that "could" support them year around is anything south of the southern border of North Dakota in the US. There are many news stories regularly reporting that piranha are being caught in American freshwater lakes and rivers.
The story that lead to my brief investigation was a single paragraph story of a 12yr old boy catching a piranha in the river that runs through downtown Kansas City, MO during the first week of December with ice chunks floating by.
As an aside, at one time I had a "school" of 50 silver dollar sized Serrasalmus nattereri in a 500 gallon tank at one time. They would devour ANYTHING I put in the tank with them. Crawdads, turtles, bass, bluegill, etc. The "school" slowly dwindled down to a single, 12" fish I named "Holmes" who died in a power outage the winter of 1985.