Yes most fin boxes are surrounded by hard foam. But not all! So If you suspect that the fin box is cracked and the hard foam is not then you still have a water tight area but with a hard hit that this board took I would still do the bubble test and see if the box and surrounding foam is cracked. If the hard foam is intact then the repair would not be that expensive because you just router out the fin box and replace the box into the hard foam and then glass the top and bottom and paint. With the pictures that have been posted I would bet the hard foam is damaged. But the bigger issue is that the guy should'nt have sold the board to JD in that condition! So I hope he gets his money back.