20 years ago I had a 36 foot fiberglass boat that had a balsa core deck. A large area of deck was soaked and deteriorated the entire length of the coach (about 15'). The whole area was spongy and flexed every time I went forward. I drilled some 1/4" holes on the underside and water ran out. I removed and re-bedded the lifeline stanchions.

Now, the rather non excepted or recommended, nor the proper repair but,,,, I drilled a lot more 1/4" holes through the underside of the deck about 2 to 3 inches apart through the entire area. I bought 4 cans of that canned foam for weatherproofing and filling gaps, and slowly squirted foam into the hole. Water poured out of the holes ahead of the flow of foam. Following the water, unset foam started coming out of the holes. I'd let it set, move the tip and squirted foam into new holes repeating the process working my way to the end of the area. Made a bit of a mess but it displaced the water and made the deck rigid.

I had a bit of a problem in one area where I put too much foam in and it raised the deck. I ran out and started jumping on the deck managing to get it back as it pushed more foam through the holes below.

Anyway,, It was fine with no sign of deterioration more than 2 years later when I sold the boat.

Make of it what you will.