jackforbes said..
Fixed my cracked surfboard with expanding foam filler and gaffer tape. Took 5 mins and still going strong!
Tried to fix my sons boogie board with quick grip only to find it melts the foam, so then tried expanding foam has some solvent which also eats into the foam although it expands into the dissolving foam cavity so kinda works but the result is a bit ugly. So your surfboard probably has more expanding foam than you may have bargained for...