If your iPhone 15 has fallen into water, the first step is to immediately power it off and not attempt to turn it on again, as this could cause further damage. Next, you should place the phone in a bag of uncooked rice or silica gel packets to absorb the moisture. It is important to leave the phone in the rice or silica gel for at least 24-48 hours to ensure all the moisture is removed. After this, take the phone to a professional repair service or Apple store to assess the full extent of the damage and complete any necessary repairs. It is crucial to act quickly and not attempt to charge or turn on the phone before it has been properly dried and assessed by a professional to avoid causing further damage.