There is little reason for shells to dissolve in the ocean.  Ocean water is
fuly saturated with calcium carbonate.  It really can't hold any more.  
Shells are in thermodynamic equilibrium with the ocean at ambient
temperature as far as that goes.  On the other hand, freshwater shells do 
dissolve quickly if the periostracum is abraided or lost.  I suppose we all
have examples of this in our collections.  It is most obvious in the
missing spires of freshwater snails.
The protein matrix binding the calcium carbonate is another story, of
course.  The matrix is 2 to 4% of the shell by weight but it is not in
equilibrium.  The snail apparently continually repairs this.  Vermeij found
the shell loses half its strength withing mere hours after the death of the
animal (if I remember his numbers correctly)!
That is far too short a time for cracks to develop (and, in any event, the
cracks run through the matrix and do not attack the calcium carbonate
"bricks" of the shell).