Thanks heaps for the info Scarbs - much appreciated. My board actually weighs 11.3kg's with fins. For the hell of it, I might just try and taking the fins out and compare the weights.
I suppose what I'm trying to do is find out whether my board contains water without going to the extreme of drilling holes in it etc. I've taken the bung out and everything is pretty dry at that point but the cracks were in front and back of the board I so can't tell if there's water in the foam further in from these points. I let the board dry out for a few days before repairing the cracks but now wonder whether I should have let it dry out longer. Oh life.... Probably just being paranoid.

Anyway, thanks again for the info - much appreciated!