Herman Cain goes for the extended metaphor, revealing considerable ignorance about boating and tides:
A rising tide lifts all boats. A sinking tide hurts the boats at the bottom. Dems don't want minorities to know that http://t.co/JUiULpNEUo— Herman Cain (@THEHermanCain) October 24, 2014
For the exact same reason that a (literal) rising tide (literally) lifts all boats, there is no such thing as a boat at the bottom. They are all floating at sea level. The issue is that a sinking tide is especially problematic for boats with large drafts. That's going to tend to be larger boats (which need to extend substantially underwater to avoid rolling over) and also boats that are full of cargo. There's no reason to think a poor person's boat is unusually likely to run aground in low tide. If anything, the opposite is usually the case.
(h/t to Brian Beutler)