Usually a marina will have a "waste oil" barrel. Old motor oil, oil contaminated bilge water, anything with oil in it, goes into that barrel and goes out for recycling, or use as heating oil.
"Why would pumping or pouring the remaining water out be a problem? " No problem, as long as it is WATER. but "any remaining oil residue would simply evaporate" if what you dump leaves ANY rainbow sheen on the ground, for any amount of time? Get out the checkbook, no one wants to hear about evaporation. It is still an "oil spill" and ground contamination is possible, so they don't debate it, they just scoop up the fluid AND THE GROUND BENEATH IT and send you the hazmat disposal bill.
A man in Florida let his kids play with a bottle of mercury, supposedly gotten from a potions store or found in his garage when he bought the house...the authorities wound up tearing up the driveway that the kids had spilled it on. And bringing in dump trucks. And last I heard, coupe of months ago, they were debating who would pay the quarter million dollars or so that they figured for the earthmoving and hazmat.
There are some places you just don't want to go.
Yeah, I guess you're right.
It's just that in light of things like the Gulf oil spill and sewage treatment plant overflows and such it seems kind of silly that someone can't pump out their bilge water after removing the oil from it.