We stayed at Haus Arkona for a couple of days during the last week of April and enjoyed it thoroughly. The room was very nice, overlooking the main street. Sellin is charming, the hotels and shops are almost Victorian architecture-- as befitting a seaside resort from the end of the 19th century.
The Sellin pier is just a short walk away. A boat tour is available there to tour the chalk cliffs and it also stops in Binz.
The Haus Arkona staff was friendly and most helpful.