This is originally associated with the Old Town Neighborhood, but can be used anywhere there is a large river or bay. To be honest, the river this was based on was really not that large, two and a half paddlewheelers wide at this point
The Old Town Jail is actually a large ship. It is a decommissioned warship (with all its weapons removed of course) with all but one of its masts removed (there is only one sail on the ship in case of emergency). It is several decks tall. Without much of its weight, it has a light draft (if on a river, it can normally only travel the river in the spring high water levels).
The ship smells of tar, old wood, and old blood. It moves slightly in the current where is is anchored down by four points. The smell of river plants decomposing can be strong during the day, but the breeze along the water clears that every night. The sound of a harmonica can often be heard, playing low and lonely tunes. Though a tradition of percussion (drumming) is strong in the jail, as odd tunes are played at work times or when they decide to dance and entertain each other.
Most of the time, the prisoners are kept under deck. It is a bit cramped and quite gloomy. There are no assigned places for prisoners, so they all just flop where they want. However, there are a good number of hammocks here where the weapons used to be, so crowding is not a problem. The ports have been removed of glass. They and the weapon’s slots are grilled with iron bars. The food is cooked by a trusted prisoner in the ships old galley. There is very little to do but be bored. The ship board chores of cleaning and mending are actually performed by the inmates, who all have a vested interest in the ship being sea/river worthy. The Guards (called Mates) check the supplies in and out, if anyone wants them.
To be honest, it is a fairly run place. Inmates simply serve their time and get out. The Mates (guards) are fairly relaxed, since everyone is "in the same boat". The hammocks are cleaned and rotated often. Most of the Inmates realize that keeping the ship clean and water worthy is in their own interests, so they do their light chores with few complaints. The Inmates keep their own order and most people fall into the system of the decks quite quickly. (In fact most people keep to themselves, rather than risk the rough and dangerous justice of the Inmates). Note: Mates and Inmates all chow the same food, so it is not that bad (if you don’t mind too much hardtack), and sometimes share the galley table.
There is little in the way of security on the ship, just a few Mates who man the poop deck and the undercabin. They are there to sound the alarm should someone go over (firing at them with crossbows/ rifles as needed). They also watch the prisoners when they get deck time. They are supposed to attack any unauthorized boat coming towards them, but that seldom happens. They are under the command of the most senior guard, called (creatively enough) the Captain of the Jail. The Mates are guards, police militia types, but they wear stripped down surplus navy officer uniforms.
If the Old Town Jail is a bay, the distance from the shore is all the security they need. The town rows out supplies every day or so, inmates are transferred on the same craft.
If the Old Town Jail is on a big broad river, the distance from shore is some security (as before the days of gym clubs, not many people learned to swim well). However there are two low narrow towers on either side of the bank. In each is posted two guards at most times, just to make sure nobody "gets any ideas". One of these towers have the official office of the Old Town Jail where papers are kept to satisfy the legal types.
The city side tower has a small dock for "The Key", the only ship that is supposed to go out to the Jail. It is crewed by Mates who are not onboard at the moment. They are a bunch of clumsy sailors, but they get the job done.