A dremel was used to cut out the rotten plywood.  However, the removal of a few more tiles revealed that different thicknesses of plywood had been used in the area.  The ridges had been slightly smoothed over with cement.  The thicker plywood had to be pried out and the cement removed.  There were a couple of other pieces of  thicker plywood in other spots.  A router had to be used to trim along the walls before the plywood could be pried up.  Fortunately, most of the plywood in the floor was the same thickness.