If you crack the white part of your iPhone 7 is it still waterproof

I presume you are referring to the glass face of the device. At any rate, iPhones are barely water resistant [1] in the first place, they are certainly not, nor never were, waterproof.

If the glass is cracked, or the screen was replaced, you should do everything possible to avoid contact with water.

[1] Your iPhone (ed. - iPhone 7 and later) is splash, water, and dust resistant and was tested under controlled laboratory conditions with a rating of IP67 under IEC standard 60529. Splash, water, and dust resistance are not permanent conditions, and resistance might decrease as a result of normal wear. Liquid damage is not covered under warranty.' - per Apple''

https://support.apple.com/en-ca/HT207043

What does IP67 mean?

• Dust tight - No ingress of dust; complete protection against contact (dust tight). A vacuum must be applied. Test duration of up to 8 hours based on air flow.
• Immersion (up to 1 m depth) - Ingress of water in harmful quantity shall not be possible when the enclosure is immersed in water under defined conditions of pressure and time. Test duration: 30 minutes. Tested with the lowest point of the enclosure 1000 mm below the surface of the water.