Air temperature is not water temperature
A mild forecast can coexist with cold water. Open-Meteo data on this site describes the air at the coordinates and is labelled accordingly. Use a suitable local water-temperature source or an actual measurement where available, and treat uncertainty conservatively.
Plan the end before the start
Identify a stable exit, an alternative escape, dry clothing, shelter and a way to raise help. Choose an exposure appropriate to your experience and the observed conditions. Do not rely on how warm you feel during the first minutes.
Stop conditions belong in the plan
Wind, current, surf, poor visibility, difficult access, concerning water appearance, illness, equipment failure or an uneasy swim partner are reasons to leave or not enter. A checklist helps make that decision before commitment and preserves why it was made.