What a wonderful informative and useful post,
@Marie 2!
My ocean water isn't cold so I haven't run into that problem
I can add a bit of information, as I've often ran into Libres being too cold.
It's not the reader/phone being too cold, it's the sensor itself when the back of your arm gets cold. In my case it has happened at campfires (hot on the front, cold on the back), while doing manual work outside, making me warm enough to work with little clothes in a cold environment, and also when swimming.
When swimming is the hardest to solve, as there is no place to warm the sensor. Campfire is the easiest, just position that chicken wing like it is a chicken wing.
In all cases, it takes quite a while for the sensor being warm enough again to give readings.
Edit: fun but off topic fact: in winter I can't use my meter at night without warming it because it's too cold in my bedroom. Scanning the Libre works just fine, as the sensor stays under the blankets when it's cold!