Hi, my advice is take her out of the water
..then u can start grinding ..from the inside if you can't find anything on the outside...looks like a badly done repair job..something copper underneath which bled through the laminate?
look and see when on the dry.
I had a similar problem on an older racer
..the yacht had a speedo sender (Doppler) glassed into the hull
..and through time it started to decay..looked like a boil..
Is your hull
a solid layup
or has it a core
? hopefully not a Balsa core
which tends to soak up water and if there is some copper fitting it may just corrode away ..so someone put a patch of glass over it?