Great move by the Pats to sign Waters on multiple levels in my opinion.
Waters has already proven that he is more than adequate at the position. While the KC OL did have some issues, my understanding is that it was more on the outside at tackle than at guard. At the bare minimum this gives the Pats a legitimate starter while Connolly's ankle heals.
If (as I expect) Waters is an upgrade over Connolly it obviously makes them better at RG. But I think what many are overlooking is how very much this improves the OL by making Connolly the primary backup at LG, C and RG. An unfortunate reality is that injuries can and do happen in the NFL, especially to linemen. I have all the confidence in the world in Connolly coming off the bench at any one of those three positions; I wouldn't say the same thing about Wendell or Ohrnberger even if they were healthy.
While this move may be disappointing personally to Connolly, his reverting to that primary backup role, à la Russ Hochstein if you will, is what I've been hoping would eventually happen all off-season. This signing upgrades two roster positions, starting RG and backup interior OL.