Air Temperature Lies
Your weather app says it is 15°C (60°F). That sounds pleasant. But that assumes still air. You are a wing foiler. You exist in moving air.
Evaporative Cooling
When you are wet, the wind evaporates water off your skin. This process steals heat from your body rapidly. The stronger the wind, the faster the heat loss.
The Real Feel
In 20 knots of wind, that 15°C day will feel like 8°C on your wet skin. If you dress for the air temperature, you will get hypothermia.
The Neoprene Barrier
Wetsuits work by trapping water. However, you also need "Smoothskin" or rubber panels on the chest and back. This rubbery material sheds water instantly so the wind cannot evaporate it. Standard fabric-covered neoprene soaks up water and acts like a refrigerator coil in the wind.
The Cooling Effect:
Dry Skin in Calm Air
Heat radiates slowly
Wet Skin in Strong Wind
Rapid heat loss through evaporation
Summary
Ignore the thermometer. Dress for the "Real Feel." In strong wind, wear a thicker wetsuit than you think you need.