Try tapping the hull adjacent to the opening. If the balsa very wet or rotted you will hear a softer sound so you will get some idea of how extensive it is.. A moisture meter is a better indication. Also you can drill into the core (possibly from underneath) to see how bad the balsa is.
Is it possible to repair from the bottom. I just cut out the inner skin and removed balsa in a section around my port chain plate today. Starboard was redone with very good results 2 years ago.
A good surveryor might take a look for free.