Sailing has been in the works in Oldschool Runescape since mid-2023. We're now in early 2024, with no precise release-date announced yet as far as I'm aware. The wiki, as of the creation of this market, claims without citation that "Sailing is expected to be added to the game later in 2024, or sometime in 2025". But that's not very precise!

Market resolves Yes if the Sailing skill is added to the game—in the sense of full release, not just in e.g. April Fools semi-release or beta-world test-release or something—before the end of 2024. Otherwise it resolves No.