A semi mobile fortress placed in the mouths of rivers, bays, and other bodies of water up to several miles off shore
"Deepbend, the toughest place to live south of the wastes. The inmates are tough, so we gotta be tougher. They may complain of 'abuse' and 'cruel and unusual punishment' but you know, they deserve every last kick"
-Darner Gearbend, Warden.
The Rock of Gibraltar, once a stalwart of British power, the gate to the Mediterranean, now is a fortress for the Eurasian Alliance. It's fair teeming with neo-Soviets and submarines.
"Ye thought the orbital penitentiary around Venus was hard ta endure? That place is a vacation spot compared to Davy's Lockdown! If the guard borgs can't find an excuse ta peel the skin off yer bones and the inmates don't use ya for their bettin pool, the pressures o' bein that far beneath the waves will drive ya to the brink, mark my words!"
An underwater prison with a dark past, suitable for any sci-fi, neo-dystopian, or cyber punk setting.
It is said in the hidden myths of the banned and hunted Cult of Ma-O, that the Seamus Straits are where Ma-O fell when the god Jove hurled him from heaven. His physical body was smashed into pieces of amethyst and his spirit trapped within the warm waters. Here under the waves, once when built on dry land the pride and joy of the De Madden Company, Castle Seamus is now occupied by a unique underwater tribe.
There is a person who has a well-known reputation, probably involving something illegal. But this person is either very clever and crafty, or really like his or her privacy, for no one knows who this person actually is. As an added twist, perhaps this person is of an age that would noramlly be wildly inappropriate (say, a child is a much-feared bandit) or maybe not the expected gender or even race.