Amazon offers this heavy plastic bag with a locking seal to let you use your phone in the water. Ziploc has long been my favorite beach case for my phone and kindle, wonder how they compare?
This less than $3.00 plastic bag does a great job of keeping the sand and sea water off (or out) of my iPhone 7+. You can easily use the touch screen through the clear rubber surface, and take photos through the material.
Alternatively, there is another better-known brand of plastic, well sealing bag that has served me well for years: the Ziploc Freezer Bag! I use them for sous vide, kids lunches, throwing crap in the freezer, and reading my kindle in a hot tub.
Comparatively, the AmazonBasics is purpose built and will take more impact and abrasion than a Ziploc. The Amazon also has a handy lanyard, the only way I know of affixing the Ziploc to me is duct tape (duct people, duct.) I think the Ziploc freezer bag more reliably activates the button-less haptic button on my iPhone 7+, however.
The price seems to float around on the Amazon bag. For $3 it is worth having around if you are going to spend a lot of time at the pool or beach.
AmazonBasics Universal Waterproof Smartphone Pouch
Ziploc Freezer Bag, Gallon, 28 Count via Amazon