Prison ships were usually old dismasted “hulks” that were anchored in harbors. Hard to escape from, impossible to tunnel out of.
During the American Revolution, the dumping of convicts sentenced to "deportation" to the colonies was restricted, probably because "why add to the enemy's numbers?"
One exception was screened ruthless convicts/deportees recruited to be British "mercenaries" in the Western, less-populated parts of the thirteen colonies. More often than not, they were just sadistic raiding and pillaging bands intended to punish the rebellious colonists.
Any way, all the deportees were piling up in vessels moored in the Thames, and one or more caught fire.
That spurred the push to dump them in Australia as indentured servants to the settlers. In general, the Scot cons thought it was great to get as far away from the English king as possible. However, the Irish cons carried with them that big chip on their shoulders.
There was one in San Francisco harbor back around the 1850s.