Hi Fernando,
I could easily see the cannon ball 'conversion' being used as a 'ball and chain'. Smaller 'balls' were used whereby the prisoner (POW) could be used for 'labour' with armed guards 'overseeing' the work.
Coupled to a short chain (so the prisoner couldn't pick it up and run) , it would be difficult to escape. Onboard a 'prisoner ship' the prospect of trying to swim to 'freedom' with 8lbs+ dangling from one leg..... well I think you get the idea
All the best
David