Putting electronics in water isn't a good idea. The S5 is rated as water resistant, not water proof. Great protection against accidental exposure to fresh water. If the battery cover is slightly ajar or anything is loose, water will get in. There is no immunity against doing ill advised and inherently hazardous things.
Anyway. Genuine phone or not, it sounds like the OP got water in his phone. First, take the battery out and do not try to turn it on every so often to impatiently check whether it has recovered. Every time you do, you run a real risk of permanent damage. At the very least, you need allow abundant time for it to dry out, which will be a long time if you don't disassemble it for decent access /airflow /allowance for evaporation.
Also any professional repair would also include, at a minimum, gently scrubbing the electronics with a solvent to remove mineral deposits from the water. It is very common that a phone will have serious issues even after it has 100% dried due to conductive minerals left behind after water exposure.
On the positive side, phones will usually survive water immersion with zero to minimal permanent damage, provided that you resist powering them before they are cleaned and dried. If you are proficient with electronics, give serious consideration to disassembling the phone and cleaning /drying it properly. You would need a heatgun to detach the glued in place screen assembly. Most components are not accessible without this preliminary step.
If disassembling the phone sounds daunting, then you should get a professional repair. But at the very least, allow the phone to dry a considerable time before trying to turn it on again. Impatience may well result in permanent damage. Placing it in a sealed container, with silica gel to remove water vapour would help dry it a lot faster. Rice would be a distant, distant second and considerably less effective approach. Do not put the phone in direct contact with loose rice. Just put the rice in the same container.
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