Summer is nice and warm. High season a short is ok, a few weeks with just shorts is possible. Usually a wetsuit is always needed. Water max 22°C in summer, normally around 15°C. Winter gets cold, but a thick wetsuit, gloves, mask and shoes make kiting almost year-round possible. Air: summer between 20-30°C, spring and fall around 15°C.
Take A7 to Flensburg. Take the last exit before the border to Denmark and follow signs to Niebuell/Dagebuell. Get to the harbor of Dagebuell and take the ferry to Amrum (takes about 2 hrs). The beach is directly next to the harbor where the ferry arrives.