A marge ship, like an ocean liner, may be incredibly heavy. But the weight of something isn't the only thing that will determine if it sinks or floats. Buoyancy is also very important. Think about when you're in a pool and you hold your breath - you can actually float on your back for a little while. this is because the air in your lungs is providing you with buoyancy - essentially pushing you up out of the water*. Well, ships operate the same way - the inside of a ship is mainly hollow and filled with air, giving it the buoyancy necessary to float.* - Coincidentally, this is how hot air balloons work, too. The hot air in the balloon is lighter than the air around it, and provides buoyancy. Science!