The sea is warm, prices are lower, and the phone in your hand gets so hot you can't hold it to your ear. And if it were just a hot case, that would be one thing — but that is where a far less pleasant story begins.
Lithium heats up while it is working, and in direct sun the temperature climbs without stopping. First the iPhone brings in its protection: brightness drops, charging slows, and there is an empty bar on the screen. Then it shuts down. For the battery, every episode like this is like a tan that will not wash off: capacity fades, and one day the battery swells. By then a lifted display and a cracked back panel are no longer a hypothesis but something lying in your hand.
The danger is that there is no loud boundary between "it just got hot" and "time to rescue it". Some models carry on for months after overheating as if nothing happened. And at New Year, once the warranty has run out, they start shutting down at low charge — and by then it is too late to wave a towel.
What actually helps: keep the phone out of the sun at the beach. Do not leave it on a towel or under a car windscreen. Do not run heavy games or navigation in direct sunlight. And do not charge a device that is already hot — that is a fast way to finish off the battery.
Our technician in armour under the southern sun looks at such phones with restrained sympathy: many summer battery problems are fixed by a couple of minutes of prevention. True, those who learn from their own experience end up coming in later with a swollen case.