Matt, on the coaming I use the white composite (plastic) Azek like trim boards found at Lowes and HD. It is a little softer than hardwood, but machines well, and takes paint well, and won't rot or split. On your last picture I am guessing you are referring to the shear clamp up under the decks. If this is rotted, I fear you have a bigger project. The other fiberglass must have been compromised or the hull was allowed to fill with water. If just a spot, then it can be addressed locally. You will have to cut back the deck, and the hull in that area to access and replace the damaged wood. If the plywood deck and hull are still sound, you can put the cut out pieces back and install them with butt blocks, plenty of epoxy or adhesive, fill the kerfs and glass over. More pictures always help tell the story.