sea legs 
plural noun (one's sea legs)
A person’s ability to keep their balance and not feel seasick when on board a moving ship: now I’ve found my sea legs I’ll be much more useful...
- For the uninitiated, this book is the closest thing to getting one's sea legs without ever having to leave shore.
- She had had her sea legs for too long now to be getting seasick.
- The three stepped ashore on Trinidad, and tried to adjust their sea legs.