I think in this specific case, it was written this way both to allude to the slanginess of Huck Finn, and because it sounds good due to "boat don't float" rhymi

I think in this specific case, it was written this way both to allude to the slanginess of Huck Finn, and because it sounds good due to "boat don't float" rhyming. To me, it has a similar pleasant-sounding impact as alliteration.