I broke the screen on my s9+ phone. If I but a replacement screen kit, will it still be waterproof?

Water resistant (not waterproof) & it may depend on which. I think you should consult with Samsung if you want to maintain that.

The phone is water resistant, and was never advertised or reported to be waterproof. So unless you replace the screen in the exact way the original was installed, with an OEM part and proper seal, it will no longer hold the water resistant spec. Now if you have Samsung or a qualified Samsung repair center does the replace, your warranty will be void if you do it. So even if there is a warranty issue that has nothing to do with YOUR repair, that will be blamed as the cause.

Definitely not. These phones are just barely waterproof as it is out of the factory, which is why they do not guarantee it.

I doubt it.