I don't know what model you have, but I cut a 1" hole in the stern end (stringers) of my 23. Inserted a piece of pvc pipe through hole and was able to pull samples from the foam. After about 1 foot, the foam came out dry. I plugged holes with rubber plugs. If I would have continued to get water, I would have probably kept going.

I have an 89 20sf. I already cut off the last 6" of both stringers to rebuild the transom from the inside. The starboard stringer was soaked but the port was pretty dry. But both stringers are leaking towards the center under the console.

My 1973 20sf was wet too and it has been out of the water for 10 years.

We also found the forward fish box through hull drain was letting water into the starboard stringer.

Check my build thread for pics. My deck is out so we cut the tops off the stringers and removed the foam.

Frizzies solution is a good one, and safe.

FYI, I cut the side out of almost half the length of the main stringer in a Seafari 25. It glassed back together just fine.