At worst, cut a 6” or 8” inspection plate hole in that area, air it out WELL & weep Glu-vit or West Systems Flex 650 into that riveted area. Best repaired from the inside, at least from what you tell us via cyber-space.
And ANY ventilation to the bilges a good thing, so periodically open that hatch up and let the hull air out, and that’s how I would leave it when not used (if area has a bimini or whatever) or whilst stored for the winter.
On an frp (read: plastic) boat w/ wood stringers, I once stuck ahairdryer on ‘warm’ into such inspection port to air out and dry out the hull ... do NOT use it on HOT! Are you want to do is get warm air moving, and warmed tin will accept the glue better. Read my posts on using that Flex 650, amazing stuff and a kit is only $20. Plus it is flexible, whereas Glu-Vit is not.