Ziplock bags are an excellent inexpensive waterproof/water-resistant solution to your problem.
If it happens that you drop your phone while in the bathtub, yes, the bag will make your phone safer, but your phone will be protected for a few moments.
So be quick to take it out from the water.
How do I keep my phone dry in the shower?
Suggested clip 112 seconds
How to: keep your phone dry in the shower. – YouTube
Start of suggested clip
End of suggested clip
Can you bring your phone into the shower?
The best iPhone 7 feature may be its scariest: Apple’s new device is water-resistant, meaning you can take that sucker in the shower with you and scrub away at the ever-thinning line between man and machine. I tried it and can confirm that the phone will withstand a continued stream of water.
Is a plastic bag waterproof?
Just look at the numbers: Plastic grocery bags cost pennies to make and hold more than a thousand times their weight. They’re light. They’re waterproof.
Can Ziploc bags hold water?
Freezer ziplocs handle boiling water just fine and don’t get holes. The storage grade is intermediate. It works much better than sandwich bags and not as well as freezer bags.