Nuclear submarines cost on the order of $6B a pop, so I'd hesitate to describe anything about them as "cheap". We use nuclear reactors in them because there's no other power source that's as compact and self-contained.
A (previous generation) terrestrial plant already has systems that can dump ludicrous amounts of water onto the core, but they all rely on there being power to operate pumps. A nuclear submarine that can dump in sea water does not need this - the body of water it is in is cold enough (relative to the heat in the core) that it will naturally carry heat away.