Illegal to pour any container of urine overboard.
Federal law makes it illegal to discharge "sewage," defined in the CFR as "human body wastes and the wastes from toilets and other receptacles intended to receive or retain body wastes"...and "discharge" is defined in the CFR as "includes, but is not limited to, any spilling, leaking, pumping, pouring, emitting, emptying, or dumping."
So while it's legal to urinate, or even poop, over the side directly into the water or while in the water...if you put it in any container first---even a coffee can--you cannot legally empty that container into any inland waters or coastal waters within 3 miles of the nearest point on the whole US coastline.
I'm just the messenger, btw. The logic escapes me too.
Am I right in guessing that you've installed an Airhead and were hoping you could legally dump the urine jugs instead of having to carry the d'd things ashore to pour 'em into a toilet?