That entirely depends on how big the crack is. Most of the time , if it is small cracks near the middle, you'll be okay. If it is spider web cracked on the screen, it is going to be compromised. Water proofing is around the edges of the screen using adhesive, if the actual screen has cracks, water can penetrate into the internals of your device. Apple does cheap replacements for these devices. iFixit also has great replacement parts. You could also just add some clear tape of the screen if you don't want to spend that much money.