Co To Island (Đảo Cô Tô) in Quang Ninh province is a remote small island. It could be a good alternative for overcrowded Ha Long Bay in Gulf of Tonkin. The only way how to get there is take a boat in Cai Rong city or in the Quan Lan Island.
From July 15, 2016 foreign tourist are allowed to visit this island without any permitt. Read this article about this information (I recommend to check this information before you come to this island). You will find there some beautiful beaches with clean ocean water in Co To Island. There are also restaurants, waterpark, accommodation is possible in hotels.