Can be very tough in onshore winds to get out, due to the shore break. From 3-4m big waves with lots of power to small choppy waves. 3m tide. Be aware of long rock lines (ca. 20m into the sea) every 100-200m from Westerland to Kampen. You can't see them while high tide! But they are marked with a sign on the beach. Strong 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 to the North of the big building on the beach is one access. The other is South of the buildung, at the Seeblick Restaurant.
In summertime it should be avoided! Flying kites is forbidden anyway and can cause problems. During summertime lots of tourists, so take these spot in summertime: Rantum K4 or Sansibar. Fee for accessing the beach. Tourists have right of way. Onshore wind is a no-no. At the beach of the Seeblick restaurant, talk to the guys in the restaurant first before going on the water for additional info.