You are the loading officer on MV Puffin Lass. Pick a vehicle and tap a space on the car deck. Watch her sink lower in the water and lean if the load is lopsided. Keep her above the Plimsoll mark, then ask to sail.
A ship pushes water out of the way. The water pushes back up. She sinks until the water she has pushed aside weighs the same as she does. That is Archimedes' principle.
Every lorry you load makes her heavier, so she sinks a little deeper until she has pushed aside 44 more tonnes of water. Puffin Lass sinks about 1 cm for every 15 tonnes.
Sea water has salt in it, so a bucket of it is heavier than a bucket of river water. A ship floats higher in the sea. That is why the Plimsoll mark has an extra F line for fresh water.