Is this normal on older boat or does that mean the core has got water?
It means the fiberglass
has a high moisture reading. If the core was rotted (as in high water content) that issue would be possible to determine. Drying out a bottom may be required. You can have the bottom evaluated but you can't always know why. A limited survey
is always possible if you ask for one up front. It would usually involve the time required plus travel both ways. If you asked for a full survey with this item checked first you can usually ask for the surveyor
to stop the survey with no written report at a reduced fee. I would do that and if the results are acceptable or nearly so continue with the survey and latter decide if you want the boat or not.
There is a limited amount one can determine with a moisture reading and good thumping. Visual inspection
might indicate something you might not notice but you won't get much more. Surveyors don't have xray vision. They can spot warnings you might not see and those are what you can expect from a good surveyor. You could buy the boat and still have problems. A survey is not a warranty but is good insurance