We know that Kairoseki nullifies the Devil Fruit powers and sea immobilizes the Devil Fruit users. So hypothetically, what will happen if a Devil Fruit user tries to swim while he/she is in contact with Kairoseki?
They can't. The Sea Stone drains the Devil Fruit user's body of energy rendering them unable to move (as if they were paralyzed), and negates their Devil Fruit powers (but not the cursed side effects). This is the same effect as if they were exposed to the ocean. Since they can't move or breathe underwater without special assistance, they will ultimately drown.