Oh, I know. Would just be kinda cool to be in the pool with my phone.
With a good chance for you to kill it.
I have a Galaxy Rugby LTE/Pro and an S4 Active -- forbearers of the S5's back cover seal. Both can take more outdoor abuse than many other phones, but neither should ever be considered for use underwater. Your S5 is likely more fragile than them.
It's maximum rating when fully sealed is only for in-situ at a maximum of 1m.
Any extra pressure exerted on the seals when moving the phone through water can kill it. Any bit of dust or lint interfering with the seals can kill it once enough water enters such points of failure..
This phone is not waterproof nor intended to be. It is water resistant to reasonable limits of its specification. When sealed it should be alright for light rinsing, rain, splashing, or a short and non-jarring dunk in very shallow water. Do not ever consider it for any submersible use. Your ~$700 to soak away how you may choose....