If you are not all that familiar with penal history, you may not know what a prison hulk is.
Simply:
A prison hulk was a decommissioned ship, moored, and equipped (for the most part) to house criminals.
Longer version:
When Britain warred with the colonies in the late 1700s, they could no longer send their criminals there. Somebody suggested they turn decommissioned naval vessels into a sort of holding ground for those prisoners sentenced to transportation, where they could wait until space was available on a prison ship bound for a penal colony.
This was supposed to be a temporary measure but the prison hulk system lasted about 80 years. Eventually, criminals were sentenced to the hulks instead of just sent there to wait until they could be transported.