When you put a ship in the water it displaces an amount of water equivalent to the ship’s own weight.
If you put a 6000 tonne ship in the water it moves 6000 tonnes of water. When you try to move the ship forward it pushes some of the displaced water ahead of itself which causes a bow wave. This bow wave carries some energy away from the ship in the form of kinetic energy.
To get over this you either have to reduce the bow wave, by making the ship more hydrodynamic (is that a word), or provide enough energy to push the ship over the wave and onto the “plane”. This is similar to what you are seeing when an aero plane breaks the sound barrier and produces a sonic boom. It has gone through its bow wave of air.
Without wanting to go into too much detail with the physics and equations of it all… Water is much denser than air and requires much more energy to displace but the ship only has to worry about 2 dimensions as it can literally go over its wave. However, the amount of speed and therefore energy needed for a large ship to push over its bow wave and reach the plane is colossal and would mean that the ship would be far less efficient than either a truck or a plane.
Therefore the ship moves slowly, pushing hundreds of tonnes of water ahead of it, but more efficiently than trying to achieve thousands of knots.
By the way, this picture shows a small speedboat on the plane; i.e., it has no bow wave going ahead of it. It is skipping across the top of the water and riding its wave.
