When he filled in my transom prior to installing bracket, Don Herman said it was the driest transom he'd ever seen in a boat that old, so I haven't had the pleasure of a transom replacement!
If your transom doesn't flex when you stand on the cav plate, you can probably let it go till it starts flexing. Maybe squirt some alcohol in the holes to dry it out and follow up with some Git-Rot. I did that years ago to fix some wet wood around the bait well inlet/exit, which is one area you should definitely check if you haven't already done so. The polyester putty used to seal that area tends to crack, so that's a potential transom soaker! The seam under the aluminum trim around engine cut out was also filled with brittle polyester putty so I ground that out and filled with Life Caulk when I got the boat in 1975, so I'm sure that also helped keep it dry.
When/if you do a transom repair, Don also recommends replacing from outside - leave about 2" of original skin all the way around the edge so you have a reference surface to fair the new skin to. A Seafari is no different than a CC if you go that route.