Drilling lots of holes and filling them with thickened epoxy did the trick for me. Epoxy might provide more rigidity than foam.

You can see the holes here http://dilworth.org/archive/boat/haulout_images/images/IMG_0978.j...

More boatyard work here : http://dilworth.org/archive/boat/haulout.php

:: I'm rather sure my deck core is rotten. I know that the
:: best way of fixing this is to cut the deck skin off to
:: remove rotten core, replace the rotten core with fresh
:: wood, then re-attach the deck skin. That sounds like way
:: too much work for a boat that I got off of craigslist
:: for $1200, and honestly, it doesn't sound very
:: necessary.
::
:: I know a lot of people think they can put unthickened
:: epoxy into holes drilled into the deck, but because
:: epoxy runs all over, I've heard this isn't very
:: effective.
::
:: What if I pumped expanding foam from TAP Plastics or the
:: aerosol foam insulation from Home Depot into a grid of
:: holes drilled into the deck skin? Once each hole is
:: filled, I can then fiberglass over the holes. What's
:: wrong with that? If such foam is too weak, what about
:: epoxy thickened with microspheres? Sounds like it makes
:: sense, but evidently nobody on the interwebs has done
:: this and posted about it, so I'm wary of trying to do it
:: myself.