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") Furthermore, a deleted scene from ENT: "Fallen Hero" claimed that Risa had a gravity similar to Earth's at sea level.