sea legs

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun plural The ability to adjust one's balance to the motion of a ship, especially in rough seas.

from The Century Dictionary.

• noun Legs suited for use at sea: a humorous term implying ability to walk on a ship's deck when she is pitching or rolling: as, to get one's sea-legs.

from the GNU version of the Collaborative International Dictionary of English.

• Sailor's Cant Legs able to maintain their possessor upright in stormy weather at sea, that is, ability to stand or walk steadily on deck when a vessel is rolling or pitching in a rough sea.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic The ability, when walking aboard a ship, to anticipate the motion of the deck so as to walk steadily without losing balance.
• noun idiomatic The ability to travel by ship without becoming seasick.
• noun Alternative term for surimi