They probably meant "mop bucket". "Bucket" is used as an adjective here to keep the flow of the sentence and to fit the original dialogue of the panel. Like "I've splashed her with a pot of tea, and a pot of coffee, and a bucket of water today." would take up too much space. Tho "mop water" might've made more sense, then again, I'm not a translator, so idk if that was the original dialogue.
This might be an American-centric thing, so I have no idea about the differences of English in other countries, but most people wouldn't say "bucket water." Instead they would say, "I've splashed her with tea, coffee, and a bucket of water" to specify the origin and amount of water. If the intent was to specify the uncleanliness of the water, "dirty water" would make just as much sense.
The trouble is it's a parallel construct, and "bucket of water" breaks it. You would pretty much have to extend it to "a cup of tea, a mug of coffee, and a bucket of water" for the parallelism to stay, and since the focus is on the fluids and not their containers, that seems too much.
