This is a topic that I keep meaning to write an article or post about because I am someone who wants to have my phone with me (for safety and photos) all the time when fishing. Including in a tube/kayak/boat.
I have essentially been using a setup like the YakAttack version above for a few years now. In terms of what your phone is worth, the $16 they sell for is probably a fair price. It also includes a coiled tether, which is what you want in a setup like this, IMO. There are other similarly priced "fish" brand companies with their own version, but you can also get a generic brand for half price like this one.
In use, they are a little annoying because the silicone "cage" can get pulled off the phone's corners as you take it out of a pocket. And the various straps can get in the way of your phone's camera lens, especially if you have an ultrawide option. But with the coiled tether and a couple little carabiners clipped to your PFD, it has worked fine for me and kept my phone out of the drink more than once. You can see the tether connected to my NRS Chinook PFD in this bass selfie.
All that said, I have recently picked up this cheap case that has a tether attachment point built in. I plan to use it just like the YakAttack cage style one with the tether and carabiners. But hopefully it will be a more secure (and less photo blocking) option even if I'll have the annoyance of switching my phone cases every time I want to use it. If you don't mind spending a few more bucks, @Tim L's suggestion about the OtterBox cases with their built in tether attachment point is a smart way to go. OtterBox makes good stuff and there's likely an case option that would work for both everyday pocket carry and fishing uses.
for zip lock bag) placed in slide zip pocket, vest or chest pack.
higher depending on model