It doesn't make sense in a car, because people in a car aren't standing up. 'Finding your sea-legs' isn't just 'getting used to being at sea'. It's learning to compensate for the ship's movement unconsciously, by adjusting your weight in such a way that you don't feel the deck moving. When you have found your sea-legs the unmoving ground - when you go ashore - feels odd. You'll feel this after even a fairly short trip on a sea-going ferry.