I think the photos clearly show a compromised ballast/keel skin (the Contessa 26 has encapsulated iron or lead ballast).
After testing for unseen damage, remove and repair the affected areas.
The various non-destructive tests for hull
soundness, such as percussion sounding*, I/R temperature** & U/S moisture readings, can be very difficult for the layman (or inexperienced/unqualified expert) to interpret.
However, even the novice/amateur should be able to discover suspicious “differences”, worthy of further examination.
* If a Phenolic or Nylon or Urethane plastic hammer is unavailable, a white rubber mallet can be substituted. Most hardware
& big box stores carry some sort of suitable “soft” hammers. As the Captain
notes, the combination of hammer sound (pitch, resonance, & feel (report & vibration) should reveal differences between compromised & un-compromised laminate areas.
** If a hull
has hydraulic de-lamination or wet patches, infra-red imaging (or even a non-contact thermometer) will accurately show areas having a higher moisture content, as “colder“.