sea legs

Short definition from Wiktionary

Noun

1. 1
   The ability, when walking aboard a ship, to anticipate the motion of the deck so as to walk steadily without losing balance.
2. 2
   The ability to travel by ship without becoming seasick.
3. 3
   Synonym of crabsticks.
4. 4
   The ability to partially or fully inhabit a marine environment.
5. 5
   Alternative form of sea legs.