Numerical agreement for 5000 buckets of water

If the sentence said "5000 litres of water ..." we would know exactly how much water is meant and would have in mind that specific volume rather than its 5000 notional component volumes, acceptably referring to it as a singular (a volume).

However, a bucket is not a defined quantity in the same way that *gallons or litre are defined. The statement therefore does not equate to a singular volume-equivalent concept; all we know is that 5000 buckets were removed. This means that you should retain the plural usage.

From the arguments that we have already seen, it seems to me that there is no 'correct' answer. If you follow the rule that the grammatical number of the verb is determined by the grammatical number of it subject, then the verb should be plural, whether the measure be the bucketful or the litre. If you follow the rule that there is a special exception in the case where, in this case the 'real' subject is the uncountable stuff, water, then, even though that noun is not the grammatical subject, it's number has to determine the number, singular or plural, of the verb.

There is a fair psychological motive for taking this line. Water is what we are talking about and water is what is being removed.

But consider this:

Excuse me, Omnifoods, two boxes of chicken has arrived today, when I only ordered one."

This sounds decidedly odd to me, not just because nobody orders just one item from a supermarket. Chicken, in this case is not countable, since they were not whole chickens). To some it just be right. To me it sounds odd.

I want to suggest that we are here on a grammatical grey area. We have the old singular versus plural dilemma in which the speaker's choice depends on what they see as the true (as opposed to the strictly grammatical) subject of the sentence.

Rules of grammar, after all, are ways of accounting for what speakers and writers of the language are doing or thinking. They are not forms of legislation. Except in schoolrooms, of course.

So in answer to the question, I think either will do.