Hi. Can I take pics under water with my iPhone12?

No. The phone is not waterproof. It is moisture and dust resistant. And water damage is not covered by the warranty.

You can get a case for it specifically designed to be used for underwater photography, but without that keep it out of water.

You can if you have lots of money to replace it each time. It is water resistant, not water proof and the standard warranty does not cover water damage. If you have purchased AppleCare + you get two incidents of user damage and pay a deductible to replace it.

Buy a water resistant camera instead.

Definitely not. While Apple’s marketing literature states it is water resistant, they do warn against all contact with water. Damage caused by water will not be covered by the warranty.