AT WHAT WATER TEMPERATURE DOES SURFER’S EAR BECOME A REAL RISK?

While regular exposure below approximately 65 °F / 18.5 °C is the typical cause mentioned, surfer’s ear has also been shown to be common amongst water sports athletes active in warmer temperatures.  Read more here: https://surfears.com/surfers-ear-research/
Yes. Occasional exposure may slow progression, but risk can still accumulate over many years.