Some of the earlier C-22's,(including our original 1980 C-22), used plywood stringers along side the keelson. This was to make the hull stronger without having to make the hull thicker. Problem is that water works it's way into the plywood and once it gets wet, it rots. The ants probably are eating the rotted wood. Been there, done that project, very time consuming and you can never repair all of the problem because the plywood stringers are under the hull liner.
No more wood below the waterline from the "new design" boats on. Thank you Catalina Yachts!
Don