I just took a diesel engine class, so I'm no expert in these things, but Google "run away diesel engines." Wikipedia says it's rare, but..."A runaway condition can also result from oil supplied by failure of the oil seals in a turbocharged diesel engine, from overfilling the crankcase with oil, or certain other mechanical problems..." The person who was teaching that class has seen it happen a couple of times and there's nothing you can do to turn the engine off from the key or ignition wire. You need to starve it of air, which is easier said that done. I'd remove the oil. Better to never find out what might happen.
1/4" of oil will have nothing to do with a runaway engine. The oil level has to get high enough (typically several inches) to get sucked up past the piston rings, in order to be combusted. 1/4" of extra oil may contact the camshaft bearings, may cause a little foaming, but there's no way it will cause a runaway engine.
And given normal engine tolerances, and the extra leeway added for marine diesels operating at all angles and motions, the odds are 1/4" of oil will harm nothing.
Don't panic with thoughts of runaway engines, ain't gonna happen from this.
Yeah, I skipped right over that part about it being 1/4" above the dip stick line. That does seem pretty minor. But it also seems to me that foaming is exactly what you would not want to have in your engine...