Can I take photos underwater using my iPhone

No. iPhones are water resistant but you should avoid getting them wet if at all possible. resistance is not permanent.

Read about splash, water, and dust resistance of iPhone 7 and later in this support article --> About splash, water, and dust resistance of iPhone 7 and later - Apple Support

I wouldn't.

You may be able to find some cases that claim to be waterproof. The iPhone 12 and above does have a water resistance rating of IP68 that gives it a max depth of 6 meters for 30 minutes, but that makes it far from a waterproof camera. Getting service on an iPhone with water damage will likely be costly and is not covered with the initial 1 year AppleCare coverage. You may get partial coverage if you purchase AppleCare+, but it does specify "accidental" liquid damage covered with $99 deductible.

Liquid damage isn't covered - Apple Support