Oxidisation of the original bronze screws around the keel boxes seems to be the issue in the photo. Over time the whole screw can degrade and allow moisture ingress. If it is just the heads of screws, they can be cleaned and a coat of varnish, epoxy or paint – polyurethane or two-pack etc will prevent further deterioration. It would be prudent to check the screws by extracting some of them. If replacing oxidised screws, it would also be prudent to seal the holes with epoxy as the plywood may have degraded around the oxidised screws. This is a maintenance issue to keep an eye on and affects the hull fixings as well.
I hope this helps
Chris