Monarchy is like a sleek craft, it sails along well until some bumbling captain runs it into the rocks. Democracy, on the other hand, is like a raft. It never goes down but, dammit, your feet are always wet.

Fisher Ames (1758-1808) American politician, orator
(Attributed)

This is the earliest reference I can find to this metaphor. Variants:

• "A monarchy is a merchantman which sails well, but will sometimes strike on a rock and go to the bottom; a republic is a raft which will never sink, but then your feet are always in the water." This variant is often attributed to a speech in the House of Representatives in 1795, but is not found in records of Ames' speeches.
• "A monarchy is like a man-of-war -- bad shots between wind and water hurt it exceedingly; there is danger of capsizing. But democracy is a raft. You cannot easily overturn it. It is a wet place, but it is a pretty safe one." -- Joseph Cook (1860-1947) Anglo-Australian politician
• "Dictatorship is like a big proud ship -- steaming away across the ocean with a great hulk and powerful engines driving it. It’s going fast and strong and looks like nothing could stop it. What happens? Your fine ship strikes something -- under the surface. Maybe it’s a mine or a reef, maybe it’s a torpedo or an iceberg. And your wonderful ship sinks. Now take democracy. It’s like riding on a raft, a rickety raft that was put together in a hurry. We get tossed about on the waves, it’s bad going and our feet are always wet. But that raft doesn’t sink … It’s the raft that will get to the shore at last." --- Roaldus Richmond (fl. 1940) American writer. In, ed., "A Yankee Businessman in New Hampshire," American Life Histories: Manuscripts from the Federal Writers Project, 1936-1940
• "Democracy is like a raft: It won't sink, but you will always have your feet wet." -- Russell B. Long (1918-2003) American politician
• "But you have to understand, American democracy is not like the system you have. We're not an ocean liner that sails across the ocean from point A to point B at 30 knots. That's not American democracy. American democracy is kind of like a life raft that bobs around the ocean all the time. Your feet are always wet. Winds are always blowing. You're cold. You're wet. You're uncomfortable -- but you never sink." -- Colin Powell (b. 1937) American politician, diplomat, soldier