@Victor Murphy,
After a major surgery that zapped all my strength, I had that problem in my apartment in California while recovering. I was forever accidentally stepping on the raised stopper while showering and the water pressure holding the stopper down was so strong that I didn't have enough strength to pull up the stopper so the water could drain.
I tried using a plunger to see if the suction could lift the stopper up, but that didn't work. What did work was using a large sewing needle along the edge to pry up the stopper so I could slip another needle under it long enough for the water to drain out and release the pressure.