The Principality of Sealand is a micronation claiming Fort Roughs, also known as Rough Towers. It was originally an installation built during World War II by the U.K. to guard the Thames Estuary. At the time, the fort was within international waters as it was too far from the coast. Years later, radio broadcaster Paddy Roy Bates occupied the fort to set up his own station, but gave up on that and ended up claiming independence as the Principality of Sealand. These claims are not officially recognized by any established sovereign nation, however. Also, since 1987, Sealand has sit within British waters due to a territorial expansion.
To sum it up, while Fort Roughs was originally in international waters, not belonging to any country in particular, it's now located within British territory. As for Sealand's claims to independence, I'm afraid no one takes them seriously.