Seeing is believing

The shiny silver and copper device in the water bottle is called a Salino meter. It is floating in the water and the scale on top shows us the density of the water. In fresh water - the bottle left is completely filled with fresh water - the scale shows 1.000. This means that fresh water weighs 1 kg per liter. On the right however, this bottle is filled with seawater. Seawater is salt and therefore heavier. The scale shows 1.020 which means the 1 liter weighs 1.020 kg. Because of this 'heavier' water, the Salino meter doesn't sink as deep as in fresh water. She is pushed up.
And that is why a ships sinks a little bit deeper in the water when sailing from salt to fresh water. Automatically!