"Isn't it cold?"
さむい is the original dictionary form - い adjective, just like あつい hot, おおい many,あかい red, しろい white
さむく is used to join verbs in a sentence, after swapping い withく
Wouldn't "samui desu ne" be a bit more literal translation for "isn't it cold?" Why is the "-ku nai" necessary?
At a guess I would assume the ね is rhetorical whereas this is more of an actual question.