Cockpit core.

[Hardhead]

Do you know where the water was coming in? From the photos, it looks like there could possibly be some more water damage - coming from under the starboard side of the cockpit sole. I went through some of this myself a few years ago... my only good advice is - don't rub the side of your head during the project. Good luck-

[Cheechako]

-You should be able to measure some small portion of the core. My guess it's 1/2".
-As mentioned, exterior ply is fine, same glue as marine.
-Poly resin is also fine. Boats have been extensively repaired, including large hull openings, with that for decades. The boat in my avatar had the cored hull opened up for about 10 ft long and 1+ feet wide just above the water line. Core replaced and ply resin glassed. 23 years later when I saw the boat and talked with the subsequent owner there were no issues.
-I would build up to go solid/no core under penetrations like the steering pedestal. You can build up a solid chunk at home if you wish to put under there, then final glass it in. Build it up on wax paper if you do.

[coopec43]

Cheechako

+++ I'd go along with that sort of repair.

[Drew-]

Hello everyone! I hope you folks are all doing well while the world is dealing with our new normal.
I finally got up the courage to install the plywood in the cockpit today and i think it went very well. I used polyester resin with 406 thickener and put in some pieces of plywood that a local boatbuilder had sitting around. After it set up I went ahead and adhered the top skin back on with a thinner mixture. What a stressful experience at first this was. I could feel the polyester thickening every minute, even though I mixed it exactly to the specifications on the back of the container. I was way more confident the second time mixing it when I applied the top skin but ended up using way more than was necessary and it oozed out all the way around when I applied the weights to hold it down. It was no issue though, I just got some old rags and some acetone and wiped it up as it spilled. I am happy to know that the entire area is saturated and should be water tight now though(at least as much as polyester can be).
The next step will be to sand the joint down, apply CSM followed by fairing with gelcoat on top to complete the project. I know polyester is inferior to epoxy in every way but with my budget and desire to use gelcoat in stead of paint it is the best option.
Stay well!