Can be very tough in onshore winds to get out, due to the shore break. From 3-4 m big waves with lots of power to small choppy waves. 3m tide. Stronger current when its coming from South or North.
German summers last from June to September (water 16-20°C, air 15-25°C), shorty or short arm wetsuit recommended. Before and after get a warm wetsuit! Water drops to 4°C in November to February. Main season April-November, off-season possible, but very cold.
Sylt is an island in the North Sea, next to the border with Denmark. By train to Westerland. By car to Niebuell, then on a train shuttle to Westerland or to Romo (Denmark), then by ferry to List/Sylt. The ferry is cheaper, the car train is very expensive! Drive to Westerland and then south to Rantum.