Yes, the keel is hollow unless someone had poured in concrete.
There is usually a falso sole over the keel sump.
Water ingression is common and if the boat has been in northern climates, then laminate fractures due to winter freeze exacerbate the situation.
Someone had already said this and it can not be over stated that there is only one way to repair this kind of laminate damage; properly or not at all!
The water must be allowed to drain and the laminate needs drying time.
Also, are these superficial pressure-washer marks, or gelcoat cracks:
I have seen gelcoat fractures not unlike what we see above. Hope that's not the case here, which would mean the laminate has been wet long enough to lose structural stiffness and is suffering from excessive flexing.
Do not mean to scare, but have a surveyor or a fiberglass expert come over and use moisture meter and acoustical sounding, before you waste your time doing anything. You need to know how wet the laminate is and how far on both sides of the keelbox.
By the way, that's not the keel bottom "decomposing" that is filler someone had already micky moused over older damage....