Just leaving a visible sheen on the water makes you liable to a CG
fine.
But I suppose it is at their own discretion and your attitude toward
the event also has an effect on how much the fine will be..

10 years ago, my sterndrive gear case cracked and I got towed back to
the warwick river dock by the CG out of the Elzabeth river in Norfolk.
It was late at night,dark and perhaps there was oil in the water, I
dont know, but the subject never came up.

If you boat starts up leaving a visible sheen on the water, that says
to me your boat engine needs work and you are at risk of getting cited
as a polluter. They can be totally ruthless if they so wish. Better to
be proactive.

Do fines go toward funding the CG?