A line you can see
On this page11
The wording
A discharge of oil in quantities that may be harmful includes one that "cause a film or sheen upon or discoloration of the surface of the water or adjoining shorelines or cause a sludge or emulsion to be deposited beneath the surface of the water or upon adjoining shorelines".
And the other limb
A discharge that violates "applicable water quality standards". Which brings in the standards for that water body, and those vary.
Read what the sheen limb does not say
It names no volume, no concentration and no mass. The test is the appearance of the water, which makes it observable by anybody standing on a dock.
Which is the practical point for a boat owner
You cannot be under the limit by spilling a little. A very small quantity of oil spreads far enough to make a visible film, and the film is the standard.
Adjoining shorelines are in the text
So a spill on a trailer at a ramp, or on a dock that drains to the water, is reached as well. The deck is not a safe place to spill either.
Sludge or emulsion beneath the surface too
Which catches a product that sinks or disperses rather than floating. An emulsifying cleaner that takes oil under the surface has not solved the problem, it has changed where the problem is.
That matters for boat cleaners specifically
A cleaner whose selling point is that it disperses oil is a cleaner that may be producing exactly the sludge or emulsion the second half of the clause names. Worth reading the clause before writing the copy.
The consequence is not small
The placard regulation summarizes it: substantial civil penalties and criminal sanctions including fines and imprisonment. The statute behind it is the Federal Water Pollution Control Act.
And a spill is reportable
The reporting obligation that attaches to a harmful-quantity discharge is part of the same regime. Who to call and when is a question for the Coast Guard rather than for a packaging catalog.
Which is why the container is the control
Prevention is the only strategy that works against a standard with no de minimis. A closure that does not leak is the entire compliance program available to a boat owner.