A wetsuit is almost always needed. The water has around 10°C during spring and autumn, but during summer around 20°C. Air temps during summer average 18°C, but can go up to 30°C. With according gear you can ride easily from April until November, but during winter also possible with a thick wetsuit.
Mainly westerly winds coming with depressions, any strength possible. During summer lighter winds. Nice days with East wind possible, but not as often. Best with NE, E, SE and S wind.
Sylt is a small island in the North Sea, at the border to Denmark. You need to get to the island by (car) train from Niebuell, Germany or ferry from Roemoe, Denmark. The ferry is cheaper than the car train. From Westerland drive south and follow the signs to Hoernum. 100m after the Hoernum village sign turn left at the sign "Moewennest". This is about 50m before the restaurant "Theeknob". Follow the road towards the dune and park at the parking lot called "Buswendeschleife".