Re: Stern ladder
Looks to be thru-bolted to me. (IE: the transom hold-down to the lower right in the pic is thru-bolted.) It might be a pain, but get in there and see what's on the inside of the transom in that area. My SWAG is that there's an extra nut hangin' out in the bilge.
If it's thru-bolted, apply a good coating of 3M 4200 and secure the bolt. I'd probably, remove all of the hardware and seal it up and make sure everything is tight... (Mineral spirits works great to clean up 4200.)
Oh yeah, use backing washers and ny-lock nuts, stainless steel of course.