Jeff, I own a 1988 Mark II Catalina and when I was searching the 30 line-up I thought the Mark II offered the best of all the 30 had to offer (wing keel, new interior - laminants, wood backsplash, new nav. station, updated electrical panel, flat interior floor pan, new traveler, bow sprit, T-cockpit, etc, etc). Despite these updates the Mark II model before hull numhber #5397 had the standard old wood mast compression post. Check your hull number and see if its over #5397. With this particular hull, Catalina hence forth and forever more replaced the wood support with a metal support to minimize or drastically reduce the compression post deterioration and the "sagging" of the deck you are referring to. The underlying wood piece glassed into the bildge supports the post that holds up the mast. This piece of wood can and will rot over time through seepage. As a result the mast will drop a bit and cause the deck to sag where the mast step plate is located. I have a very slight sag; however, it has not gotten any worse over the years. I was aware of it during the survey, but that did not keep me from buying the boat. Our surveyor used the term, "slight settling, but acceptable". I am sure 30's in the hundreds have been sailing safely for decades with this issue without benefit of repair. Their is a thread on this repair on the 30 Catalina International Association website, if you want to tackle the job yourself. The mast of course must be down. Try and open and close the door to the head and see if their is any resistance at the top of the door against the glass liner. This resistance is always a good test to see just how much sag has occured. If you are overly concerned, check a reputable yard for the cost of repair. Overall the Mark II is a great vessel.
Good Luck
Bob '88 Mark II
Georgia