From D&D Wiki
While I would use this template verbatim for a ferret if I had to, I've actually owned a real ferret. Their eyesight is terrible, mine would literally have his nose less than a foot above the floor and be unable to tell whether it was there or not (and make a hilarious valiant leap only to look silly when he immediately flopped onto the floor). I've seen other ferrets behave similarly on Youtube and the like.
While obviously the official D&D rules don't have a variant for ferret beyond the weasel template, I would suggest taking the +3 ranks in Spot and putting them in Escape Artist (making that +7 total) because that skill is much more applicable to ferrets. They are very good at squeezing through spots you wouldn't expect, and notorious for wriggling out of their own specially-made leashes.
Apologies if this is not an appropriate discussion subject since I'm not saying the wiki page contradicts the official rules. I just figured it was worth noting that if you want semi-realistic skills for a ferret instead of a weasel, put the Spot ranks in Escape Artist. One could even go so far as to give them a penalty to reflect a ferret's terrible eyesight in trade for more Escape Artist ranks, but that might be pushing it. I would still use the weasel's +2 reflex bonus for the master when choosing ferret as a familiar.