I know for a fact that tampons don't prevent leaks entirely though they're usually small and only show up as spotting in the lining of my bathing suit. They also tend to absorb the pool water, so tampons aren't perfect, but do cups for in the water?
Also can the pool water itself damage the cup. I know that normally liquid doesn't enter the vagina, but since it's being held open a little by the cup it could happen. I swim in a saline pool, so that might not be as damaging as chlorine I guess.