Injecting spray foam or any of these other miracle cures isn't going to fix anything, you've got to cut open the deck and replace the rotted balsa with new coring, preferably foam like Coosa or Divinycell, etc. Of course you have to find where the water got in and repair/fiberglass that issue.
Not a big fan of Seadek, or any of the other "soft" (foam) deckings for big areas, don't like the grooves and also how they handle hatch openings.
Do think some of the harder (PVC?) deckings have come a long way the last few years like Plasdeck, Flexiteek, etc.