Tar's asset

• Steadiness on a ship
• Tar's underpinnings
• Good thing to find on a boat
• Nautical equilibrium
• Ability to balance on board
• Steadiness, in a way
• Ability to adjust one's balance to the motion of a ship
• Ability to walk on a ship
• Ability lacked by landlubbers

Wiktionary Word definitions in Wiktionary
n. (alternative spelling of sea legs English)