If you were to drop a lump of steel into water, you would have a big splash and the steel would quickly sink. How do steel ships float?
Have you noticed that a plastic bottle of water is heavy? What happens if you place a plastic bottle full of air in water? What happens if you place the same bottle is full of water into water?
For a steel ship to float, it needs to have a lot of empty space where water cannot get in. When a 3000 tonne ship is in the water, it moves 3000 tonnes of water out of the way.
This is called 3000 tonnes displacement. If the volume of the ship's hull is much larger than 3000 tonnes of water, then the ship is able to float.
Ships and their cargo are heavy, but as long as together they do not weigh more than the water they displace they are alright.
If water gets into a ship, the ship will become heavier. If water is allowed to replace enough of the air, the ship, its cargo and the extra water will weigh more than the water taking up the same volume and the ship will sink.