Fresh water weighs 62 pounds a cubic foot. This is the central reason that things float. Any three dimensional object will have volume. The thinner the walls of the object and the more empty or hollow it is, the lighter it is and the better it will float. This is why ships can be made of metal and have lead keels as long as they have hull contents that are not as dense as water (meaning that they weigh less than 62 pounds per cubic foot) they will float. Now, the deeper into the water you load and push a boat, the greater the pressure on the hull. This pressure comes from the volume of the water being greater for a deeper keeled boat and at 62 pounds for fresh water this adds up. This is why deep drafted boats have heavy structures supporting the skin of their hull while shallow drafted hulls can afford thinner and lighter structures supporting theirs.
The displacement of an object is the amount of water the object pushes out of the way as it floats or sinks (In the case of a submarine). Interestingly enough it was Archimedes who first discovered the concept of displacement. Though he was trying to measure the volume of irregular objects by dunking them in a tub of water and measuring the rise of the water level across the tank’s water’s surface.
Anyway, fresh water weighs 62 pounds per cubic foot while salt water weighs 65 pounds per cubic foot. This is why anything floats higher (displaces less water) in salt water and floats lower (displaces more water) in fresh water for a given weight. Basically, if you know the volume in cubic feet of an object and its weight, then you divide the weight in pounds by the volume in cubic feet and if the number is less than 60 the object will float. The lower the number, the better it will float.
A hull does not need to have any air in it to float, it simply has to weigh less than 62 pounds for fresh water and 65 pounds for salt water. A hull full of gasoline will float because gas is lighter than water this is why tankers float though they are loaded with oil or gas. One of the fairly new ideas in ship ballast is the use of water ballast or tanks that can be filled to cause a ship to drop lower in the sea and become more stable. This is also why you should respect anything containing water. Not only is it dense, but it can shift if it isn’t contained.
