According to Archimedes' principle, buoyant forces are equal to the weight of water an object displaces; therefore, solid objects and watertight hollow vessels displace the greatest amount of water when submerged, maximizing the forces exerted on the object. If the object is less dense than water, it floats to the surface until the weight of the object and the buoyant forces reach equilibrium.
Submarines use buoyant forces to their advantage. While the steel hull of the sub is more dense than the water, the shape of the vessel includes air, which is less dense than water, allowing the submarine to float in a partially submerged position. HowStuffWorks reminds readers that the sub has ballast tanks, which the crew fills with water to increase the weight of the vessel, causing it to submerge. While the buoyant forces are greater, the additional weight in the tanks counteracts these forces. When the submarine is ready to surface, the crew replaces the water in the ballast tanks with air, causing the buoyant forces to push the submarine to the surface.
In open vessels, such as ships, as long as the amount of the water the underwater portion of the ship displaces weighs more than the boat and its contents, the ship floats. However, if water spills over the sides of the ship, it displaces the air and the ship sinks. In this case, due to the change in weight of the ship by the addition of water the buoyant forces are actually less when the ship is submerged.