The air temperature lies. The water temperature tells the truth about how long your body will last. Pick a coast and a month and see what you are actually getting into.
Figures are long-run monthly averages of sea-surface temperature for representative coastal locations, rounded to the nearest degree. Actual water on any given day can vary by several degrees with upwelling, sunshine and wind. Treat them as a planning guide, not a measured reading.
Below 15°C: cold-water shock risk; a drysuit for long exposure. 15–18°C: a wetsuit; limit your time. 18–22°C: manageable for short swims, cold once you stop. 22–26°C: comfortable for most. 26°C+: warm, no suit needed. Below ~15°C, expect cramps and the early signs of hypothermia: shivering, poor coordination, confusion.