If you look into the rolls-royce.com site, then you will notice that RR offers the Trent engine also for marine application - rated at 50MW. 50MW is well over 60,000 HP which corresponds quite well with the assumption that a 747 engine produces roughly 100,000 HP at max T/O power at sea level altitude.
Remember a ship engine has no limited duration T/O power rating - it is pure continuous power - and certainly at sea level altitude.
A few other things will probably further reduce the marine Trent power output: First of all a very efficient silencer in the chimney, but also the fact that it will run on less energetic and cheaper fuel oil instead of jet fuel.
The fan is exchanged with a gear box to drive the propeller. I don't know if they ever sold a marine Trent, but a proposed application is a large cruise liner in which it will increase comfort by reducing noise and vibration compared to a traditional and more fuel efficient diesel engine.
Regards, Preben Norholm