Well, I look at it this way.
It looks like something nailed the deck like a mofo.
The "proper fix" would be to go down through all of the cracks and build it back up.
But.......The cracks may just want to come back to visit you.
So........why not try to beef it up from the bottom with a flat sheet of 1/4" plywood and matt (wetted of course) then rough up what you got and apply the marine filler (with a blade or large squegee) block the shit out of it, hit it with some filler primer, block it again and go.
To do it right would take forever and a day with no guarantees of a permanent repair, not to mention the amount of block time would quadruple.

If you go the easy way out, you're done in 2-3 days and don't paint it until you know the repair holds (couple of outings or weeks).
I would call it a professional hack job...

GT