In Short: Not Recommended
Why? Apple does not recommend intentionally using your iPhone 7 in water. They point of making it water resistant is in case you drop it in the toilet or something.
I wouldn't risk the $500-800 for using it in the shower; Apple does not cover water damage in warranty. If you need to, get any cell phone or tablet, tape it to the the glass door, and get a waterproof speaker. Or, put it in a ziploc bag, a little water should be okay. REMEMBER THESE TIPS ARE NOT RECOMMENDED.
However, Apple Watch Series 2 is made to be water proof (Not just water resistant), and is really made to be swam with and used in water.
The iPhone 7 is not made to be intentionally used in water.