How to waterproof a sesor

Hey guys, my name is LEE. I just purchased this sensor as shown in link below

I want to use this sensor to measure the water pressure, but this sensor was designed to measure the air of the surrounding. Does anyone have the idea to waterproof a sensor? I have done some research on it, and they recommend QSil 216 Clear Liquid Silicone or rubber ballon which i dont really understand how they done it. Looking forward to simple and easy understand method. Please help me !!!