You know, first thing that is critical, when you are asking for help is to know what model boat you have. You mentioned before you were looking at an ODay 26, then mentioned above it was a 25... 2 drastically different boats. The picture above looks very much like a 25.
But I'll still take a few of these...
Bottom paint? Find a soda blaster, have them soda blast the bottom... then epoxy barrier coat the boat bottom, then put whatever is the most popular sailboat bottom paint used in your area... Freshwater, VC17m is not just fast, its a popular paint.
Chainplates, are where shrouds attach, on small boats, they are usually just long flat stainless plates, that go through the deck, and attach to the bulkheads... there is also usually a horizontal plate that the plates go through at the topside of the deck... this is usually where your leaks occur, that union of 2 metals, and deck, is rarely easy to keep sealed easily. The tension/pressure on these plates, and fiberglass, cause them to stress/move, opening rigid sealants pretty easily. You'll get 100 different answers as to what is the best thing to use in that location.
If that picture above is accurate it looks like someone peeled away some FRP (fiberglass reinforced plastic) from the top of the core near the starboard chainplate.. is that correct? If so that is some pretty involved work, and since it appears the boat is in the water, I'd be scrambling to get it covered until you can get it glassed back up again.
My own FRP/core repair... what I started with:
Where I am at now... I basically have non-skid to do now.