Default Are aftermarket cases still waterproof

I want to buy a case that has a flip mirror cover. Something like the Arium or Anymode. But my question is, are these cases still waterproof? The rear cover of the S5 is probably made specifically to withstand water. I would like to take my phone into the pool or shower for up to 20min and I don't want any water damage. Any suggestions? I like this one especially by XNR