How can I fix the battery drain problem?

Hello!

I have a Galaxy Note 4 that I bought on February 2016. The phone was working greatly until last Saturday , when it suddenly died , even though the battery was at 70 % . I thought there was a battery problem , so I ordered a new battery online. I switched it yesterday and it worked perfectly for an hour or so and then it died again and it was not even charging, even though the charger is ok. I took it to a service store and they told me that since the battery was new, it was the phone that had the problem and that something causes a voltage drop , so the battery isn't strong enough for the phone to function properly. So ,here's my question: 1) Does the problem have to do with the fact that last month the phone fell in the toilet?(I took it the store and they fixed it and it was fine) 2) Is there a way that I can keep the battery's voltage high , for the phone to work?

Thank you in advance!