What you don't have, if you can't stand sailing (3,4)
I believe the answer is:
sea legs
I'm afraid I can't explain this answer.
Can you help me to learn more?
(Other definitions for sea legs that I've seen before include "Ability to resist sickness on boat" , "Stability on board ship" , "Balance on board" , "Molten rock" , "not to feel sick on board?" .)