Surely no one sends a warship "just" with their shipment of toilet acid, an aircraft carrier will be getting resupplies regularly anyway. So either the resupply run is insanely expensive and $400k is just a part of it, or someone is being dishonest and uses a cost of a single run as the cost of the clean.

The DoD will not hesitate for a second to call in contractors if they aren't confident in their crew. I'm sure they did so in this case.

Even though you're just flying a few guys (say a few acid guys, maybe an engineer who knows the toilets, just in case) out for a day or two, that's stupidly expensive. Not even just the cost of putting them on a plane or helicopter- even though you may only be paying for a few hundred man hours, you need to pay a lot to pull those guys out of whatever they were doing.

4,700 people shitting in buckets is kind of an emergency in the middle of the ocean, unfortunately. The navy is notorious for its sleep deprivation and tight schedules. Seemingly small disruptions and timewasters can lead to major problems and mistakes.

Surely you can stock on the thing.

And store it where? Dry volume is at a premium, just look at the compactness of living quarters to get an idea how little room there is to spare.

Why in the world would you stock it and use up valuable space, and fuel on carrying it around the world?

Also, it surely cannot be a good idea to keep massive amounts of acid around for non essential purposes.

How is it non-essential if that's the only way to get the toilets going?

Its non essential if you can get it shipped there easily in a resupply