Well, Moore was considered one of the best FA guards in FA, but he retired. Waters is considered as good if not better than Moore.
And I think the reason Dallas didn't address the guard play earlier in the off season is because they knew they didn't have the cap space to sign the top FA guards. They wanted to see if the combination of Leary, Livings, Bernie, and Arkin would solidify both guard spots and would be as good as the 2nd tier FA guards. If they didn't (like we see now), they knew either Moore or Waters, who were as good as the top guards but much cheaper because the NFL is allergic to old players, would be there later on in Camp to go after.