Your handset's display might have some defect and to get your issue sorted you need to get your screen replaced but it will cost around INR 17000/- if the display is defective.
may be your display connection is loose but the problem here is if you open your phone even in the gallery you will loose your waterproof ability coz even they don't guarantee it
Your device needs physical inspection. So, I request you to kindly visit nearby Samsung authorized service center. To locate the Service center, click the given link: https://www.samsung.com/in/support/service-center/
