1/2" of resin on top of the floor would be another big red flag.... no way should that be the case and yeah that would be HEAVY
the surface of the foam would likely be dry... you would need to cut out a big chunk and no water wouldn't climb IE wich up the foam... it would saturate from the bottom up over time
Again the only way to really know it to get it on a trailer and weigh it.... If it weighs hundreds of lbs more than it should then the only recourse id to start gutting it till you find out WHY it weighs so much n put it back right.
hack job rebuild, waterlogged guts.... Jimmy Hoffa under the floor... whatever