I was hoping someone else would chime in Whit, with a solution. I have that softspot in the deck and have dealth with leaks there.
My leak problems there were the joints at the cabin sides and front, mostly. I repaired those years ago by kerfing out the joint and fitting some small pieces of wood in epoxy to form new joints there. That's cured those leaks. I've also pulled the trim board at the cabin to deck connection and recaulked. In fact, I've done that twice and this second time, made a large piece that covers some discolored areas. That front cabin piece takes a beating with sun.
The last time, I almost got into fitting a beam below. I should have. There's no doubt that the cabin is holding the deck up there so the sealant will be subjected to abuse in movement.
I recall thinking I might build up a laminated beam, roughly 2" thick(or whatever the space is between deck and ceiling) and maybe 5 or 6" wide. Laminating it on a frame to a bit more camber than the deck has should leave it with a close shape to the deck camber.
I also recall the ends of this beam could fasten to the deck support structure at the hull deck joint perhaps?
I know this isn't much help. Maybe someone else has done this?