Draught or Draft
It is the depth of the hull below the waterline.
There is no gas tank on a cruise ship. There are fuel tanks, which hold the bunker fuel and they are usually located in the bilge of the ship. (Compartments below the waterline)
Ship's freeboard
A bilge is a part of a ship. A bilge is the lowest compartment on a ship where water tends to collect as it is below the waterline. Bilge maintenance revolves around removing the water that collects in the bilge. This can be done by using buckets and pumps.
It's called "draft". Draft is the distance from the waterline to the bottom of the ship.
The waterline is an imaginary line witch a ship or boat floats. The warmer or colder the water the less or more it floats. WIKI --- Waterline
70 feet
submarine
stern
Called, "draft," or "draught," this is the measurement of the part of the ship that is underwater (the distance from waterline to keel -- or the lowest point of a ship). It's the depth to which a ship is immersed when bearing a certain load; it varies as the vessel is loaded and unloaded.For example, a cruise ship on which I will be sailing soon has its dimensions listed on the cruise line's website; the draft is 28 feet, which means 28 feet of the ship is below the waterline. (That's Royal Caribbean's Freedom of the Seas which, in terms of gross tonnage, is one of 3 of the second-largest class of cruise ships in the world. The largest is RC's Oasis of the Seas and its sister ship Allure of the Seas.)
Waterline measurement is the length of the boat that actually sits in the water. For certain types of ship the waterline measurement dteremines the speed at which the boat will travel, hence the line '80 feet of the waterline, nicely making way'.
This isn't an easy question with the amount of information you have provided. Where is the vessel operating? What is its cargo? Is it operating astern propulsion to assist in its slowing. How much of the hull is above/below the waterline. How fast is the ship operating at.