Advice on Water intrusion on 330 express

If you are very sure that is the area of leakage, then I'd have the repair performed. But you must be very sure. It would be sad if you had all of that work done (gelcoat match , etc.) and the bugger still leaked. It's a boat so on a 14 year old 330, I suppose it's not to be unexpected. The good thing is that area of the hull is non-structural so it should be an easy fix.

One area on my 330 that leaked was the horizontal joint where the transom engine mount section met the vertical interior sides. There's a small 3" squarish shaped plate there where the caulk underneath it loosened up. You can't see anything unless you removed the little plates. I removed and re-bedded the area under these plates, with 3M 4200 and the leak was gone.

So, I'd say to fix that area of yours but be very sure it's the root cause of your leak, that's my two cents. Good luck!

I was told a while back by a marine surveyor that some Expresses had a quality control issue in a rear stringer assemblies. This was seen on new hulls and in most cases that he was involved in, Grady also got involved. I can't say if that is your problem but a few things would worry me. If the 'dead' sound is delamination, that is serious. Your surveyor should have and may have performed a moisture test. It is possible that a lot more area is wet than the dead sounding space.
Removing the wet stuff and replacing may not be simple at all and unless the cause of the water intrusion is found and fixed, the problem could reoccur. Repair could involved actual cutting out of the hull area, repair to the structural components and replacement/repair of the hull section.
I would recommend addition moisture testing if not done and then written quotes for repairs. It could be fairly expensive to repair.
On the more optimistic side, you could have something as simple as a leak from a scupper fitting. Depending on the moisture test results, repairiing the leak without tearing apart the hull section may be all that is needed.
One final thought, if you decide on additional testing, you should consider a different surveyor. Your current one is probably fine but a second opinion and look could be well worth the cost.
Good luck

There is balsa coring on the sides, so if this gets wet it will need to be addressed.

Do the new Gradys still use balsa coring, or is it all composite?

new ones are all composite as I understand it.