"The bird floats in the sky."
Translation:새가 하늘에 떠요.
Float, 뜨다 used here to mean in the state of suspension, is considered a stative verb. Hence, it is followed by 에, in/on.
Similar stative verbs are 있다 (stay/be), stand (서다), sit (앉다), lie down (눕다) ...
On the other hand, Float, 표류하다 used to mean drift gently is a dynamic verb. In such case, 에서 will be used to mean in/on.