I believe that what is going on here is that the salt has been used to preserve the body of Wyatt's dead wife.
Wyatt and the coffin sank to the bottom like a shot. This is presumably because the coffin was lined with lead. This was often done to prevent odors from leaking out, I believe. I suppose it might have sunk so quickly if it were filled with enough salt.
When the salt melts, then presumably the body will start to rot. As it does, it will bloat and rise to the surface. So that is why they will rise again -- when the salt melts.