It Varies by Builder "And" By Year. I Re-cored a 1967 Donzi 18 Triple Hatch, the Hull had no core, The deck was cored with Plywood. Some Builders did both (Cored/Non Cored) depending on the Buyer. Buyers could Pick core type or No Core at all. Coring actually makes a Stiffer Panel with less weight. Solid Glass will take more of a beating without failing, But at the cost of extra weight.

The 24 Superboat I used to own had coring in the deck and bottom, but not the sides. When you banged on the sides it sounded real thin. I asked the builder about it and he said that coring in the sides was not necessary.

I bought the boat when it was 3-4 yr. old, and kept it for 10 years. I tortured that boat and it didn't have a single stress crack in it when I sold it. The 24 Super is lighter than some of the other 24X7's, subsequently faster.

My 77 pantera 24 is not cored on anything but the transom.

if the deck and liner are balsa cored you absolutely need to worry about rot, balsa is wood. I have seen it scooped out like wet sawdust
Boats without wood come in many sizes, heck my neighbor just bought a 17 ft bow rider with zero wood.

All interesting replies! It's highly unlikely that I will have water intrusion on my deck since everything is well sealed and stays relatively dry. I suppose the floor could get wet eventually but to me it seems that a wood cored hull is a ticking time bomb since it takes such a beating and it only takes a small crack to let the water in and it's mostly out of sight so not discovered until a lot of damage has been done. It is good to see composites being used more and hopefully we will eventually see most boats being wood free. I thought that most 24' + boats must be cored to keep weight down. I learned a lot from this thread.

A friend of mine is a builder of the talon 22 and jcraft slalom ski boats. Unless I misunderstood him I am pretty sure he says that balsa core is stronger but foam core is lighter. I think it all depends on the purpose of the boat.
For all round non racing a boat with no wood has to be the best. ALl you have top do is replace a floor , transor or stringers once to appreciate the value of a non wood boat or a boat with a "water proof" layup. Seems to me that a current laverycraft is the cream of the crop in terms of great glasswork.