Looks like the inside of the house side with the port removed. I'd expect to see polyester/'glass - wood - exterior polyester/'glass as the structure on that largish flat area. Is that what you can see the edge of in the port opening? Is that wood core rotten too? Or is the exterior of the house polyester/'glass? If you've really got an intact/not-rotten-cored-structure then what was the removed rotten wood for? Perhaps solely for affixing the interior finish surface?
My recommendations/several answers:
1, If you have exposed core around the port opening, then clean the remains of the sealants and glues from the edges of 'glass and core, scoop out any rotted core, make sure it's dry and clean, coat with neat epoxy and fill gaps with thickened epoxy.
2, If you have solid 'glass and the rotted wood was simply for providing something to affix the interior finish then any of the suggestions above for construction adhesive or epoxy should be at least as good as the original construction which was good enough until the leak caused rot. However, I would coat out the new wood with epoxy and glue it to the 'glass with thickened epoxy. I would also coat the interior surface of the finish plywood with epoxy. I soooo hate to fix things twice!
3, If you have a 'glass exterior that was intended to be stiffened by the interior rotted plywood, which you could surmise by how thoroughly it was stuck on to the 'glass with polyester, then replace it with plywood coated out with epoxy and glued on with thickened epoxy. Polyester really does not bond wood to 'glass very well.
I've done this sort of work on our sailboats.