Unless you are planning on hibernating them (which I don't recommend), you will need a water heater, or they can get sick and die when the temperature drops too low. Hibernating water turtles is kindda tricky and a lot of people loose their turtles in the process, especially babies and juvies. So, unless you are familiar with the process, I wouldn't recommend anyone doing it, especially beginners. I don't think it's necessary to hibernate water turtles. Hope this helps!