The sea level of a planet is the average level of its seas and oceans. It was often a useful reference point for comparing heights.
After being attacked by a sur-snake on Argo, James T. Kirk and Spock were eventually found on a small island just above sea level. (TAS: "The Ambergris Element")
Enterprise NX-01 kept it's gravity at Earth sea level, unlike some Earth Cargo Service ships, including the ECS Horizon and ECS Fortunate. (ENT: "Broken Bow", "Fortunate Son")