Sounds like core rot. Water has entered the wood in between the inner and outer fiberglass. There are a lot of threads here on delamination or fiberglass deck repairs. You will likely need to get at the soft wood and remove it, replace that with new wood or other filler and then cover back up with fiberglass. Not a 'difficult' job, just a time consuming, tedious job.
Do some searching here for previous threads that cover this in detail.
Doubt the insurance will cover it unless it was caused by an incident.
Good luck
Do some searching here for previous threads that cover this in detail.
Doubt the insurance will cover it unless it was caused by an incident.
Good luck