Most waterproof-bag advice is about rain. Anglers deal with something messier: hands that are wet, slimy or covered in bait, sunscreen and bug spray, reaching for a phone to snap a photo or check the time.
Even a water-resistant phone isn't built for that. Apple says iPhone water resistance is not a permanent condition and might decrease with normal wear, that liquid damage isn't covered under warranty, and that you should minimize exposure to liquids like insect repellent, lotions, sunscreen and oil. If something other than water splashes it, Apple says to rinse the area with tap water and dry it.
Then there's the license. In California, your sport fishing license must be in your immediate possession while fishing. Some states let it live on your phone instead: South Dakota says you can show your license through its mobile app or a saved photo. Either way, the paper license or the phone holding it has to stay dry and on you. Rules vary by state; the fishing trip essentials checklist covers licenses in more detail.