Depends on how you rule it. If it's equivalent to cold-weather gear, then it's fine. It is imparts actual cold resistance and prodides soak against non-weather effects then ... well, then it gets complicated and probably needs a different guideline.
A minor ability would keep you warm in cold weather. This spell is intended to let you endure icebound water. I'm not comfortable about quantifying it in terms of Soak bonus, but it does look a bit more radical to me than "Eyes Of The Cat".
Firstly, human skin is already waterproof. ::grins:: A minor niggle, I know. As for the fat, that's a very modern view of things. We know that cats eyes have different structures, chemical make-up and neurological stuff, but Eyes of the Cat is a low level effect. I think that "being able to withstand cold water" is more of an property of seal skin than not, and the fat, in mythic Europe, is more likely just because seals are lazy, eat too much and lie about on rocks in the sun.
I think that this level MuCo(An) is actually quite a versitile level for Magi
Eyes of the Cat - See in near Darkness
Skin of the Seal - As Above
Ears of the Wolf - +1-3 to Listen roles
Nose of the Bloodhound +1-3 to Tracking by scent
Fingertips of the Spider +1-3 to climb rolls
I personally don't mind having a rough +3 as a bonus, because usually to acquire these, you will litterally gain a cats eyes etc, so the spell isn't something you'll have up when meeting Duke Tybol of the Moving Scar....