It depends on the subject. मुझे uses the oblique case: it is not the subject of the sentence. In Hindi, the water "is being liked by you". The subject is the water. So, in this instance, you need to use है.
पसंद goes with dative case, so use मुझे for what becomes the subject in the English sentence.
In the Hindi sentence, ठंडा पानी is the subject, so put है at the end.
Use हूँ only when the subject of the Hindi sentence is मैं.
Literally, By me, cold water is liked.
Mujhe (sth) pasand hai.... Mein (sth) pasand karta/karti hun. Now replace the sth with anything. In the first sentence the subject is the thing being liked while in the second I am the subject... so the endings change accordingly. Btw both mean the same.