Green’s Dictionary of Slang

the penal hulks, moored on the Thames.

1856		H. Mayhew Great World of London II 82: The cant or thieves’ names for several London prisons or ‘sturbons’ [...] is as follows:— [...] The Hulks, or any Public Works ... The Boat.
1888		Lancaster Gaz. 22 Feb. 4/3: If i didn’t know Sam was ‘at the boat’ (penal servitude) I’d say so too.