Is it safe to use my iPhone 15 to take photos underwater?

Your phone is not waterproof, it is splash and dust resistant, and even that fades away after usage. Do not ever put your phone underwater.

About splash, water, and dust resistance of iPhone 7 and later - Apple Support

iPhones are not waterproof.

That's really all you need to know about taking the phone underwater.

No! Not without a water proof case for it,but I wouldn’t do it anyway. The chance of the iPhone getting water damage is too high