The Readied Fishing Rod is an item needed to escape the H.M.P OffshorePrison's "Swimming with dolphins" escape.
The Readied Fishing Rod is used to obtain the Salmon. This is done at the southern side of the prison on floor 2, where a small jetty extends southward and Snooty the Dolphin is waiting. You will need a Plastic Red Key to enter the stairwells which lead you down to this floor. Interact with the small gate to the right of the jetty while holding the Readied Fishing Rod. This will begin a short animation; after a few seconds you will receive the Salmon.