If you are relying on a phone as your primary safety/rescue backup it would be a very good idea to consider keeping your phone on your person instead of your boat in situations where there is any likelihood of becoming separated from your boat.
With soft cases how much tension you have in the bungie or whatever runs over your case is important. If too much tension, you may get the phone thinking it is getting taps and it will launch you into all kinds of apps and modes you can't fix while you are trying to paddle (if you have a reliable lock mode you may want to try that, but will that make your phone harder to get back into usable mode on the water if you need it?). If you figure out a mount that does not run over the front of your phone that would probably be ideal. Although don't be surprised if an occasional hard splash of water makes your phone think it is supposed to do something you really did not intend.
Nelo surfskis have come with their own phone cases (at least circa 2019) and a hard plastic mount that attaches to the foot straps. The Nelo case is pretty well made (at least the phone enclosure) but the mount has proven easy to break for some boaters.