If we are talking only about water temperature then you should start wearing a shorty when the water temperature is around 72F/22C. When it drops below 68F/20C you better get a full 3/2 and so on. But this also depends on other factors, not just water temperature. Things like air temperature, weather and wind can affect your feeling of cold and you need to put on a wetsuit sooner. Check our full explanation of all things that influence your neoprene thickness choice.