What is the term for the height of a ship's deck above the water level?

The term that refers to the height of a ship's deck above the water level is known as "freeboard." This measurement is crucial in naval architecture as it indicates the safety margin of a vessel, showing how far the deck is above the waterline. A higher freeboard can signify better seaworthiness, as it helps prevent water from splashing onto the deck during rough seas or high waves.

In contrast, main deck refers to the primary upper deck of a ship that is often the main area for crew and passengers, while draft depth indicates how deep the ship sits in the water, measuring from the waterline to the lowest point of the hull. The term topside typically refers to the upper part of a ship, which can include the main deck and superstructures above it, but does not specifically measure the distance from the water to the deck.