I have the full GM Marine manuals and as Paul says they are very comprehensive.
A quick glance leads me to believe they just put bigger injectors on and stuffed more fuel in. I should imagine it is not as simple as that and might need pumps and timing breathed on as well. The experts will tell us.
The GM injector includes the pump.....hence the term "unit injector"......and it is as simple as using an injector with a bigger rating(number)......a 53 will normally have a 45 or 50 size ......so if you increase that to the N65 or N70 from a 71 series engine,then there is more fuel going into the cylinders........of course 53 s are prone to cracked heads ,so caution might be advised.......I dont think you will get black smoke like a four stroke,just more heat stress on the exhaust valves.
In the charts above they have "Continuous" which to my mind means they will run forever. "Intermittent" for occasional short periods and "Full Power" throttle wide with short life if sustained.
The engine is exactly the same as the truck engine but a lot more disaster potential in a boat so I think they are pretty conservative. They note no figures for the 6V53T "continuous" but from the other figures about 200hp seems a long life sustainable rating.