A single boat, ship, barge or floating wharf will float at different levels depending on the warmth and/or consistency of the water.
If a ship were loaded to capacity in, say, Greenland at a salt water port and sailed to a fresh water port in the tropics there would be a danger of it riding too low in the water at its destination and the risk of it sinking.
Also known as a Plimsoll mark, the Plimsoll line was introduced in 1876 in England due mainly to the work of the M.P. Samuel Plimsoll as part of the package that became the 'Merchant Shipping Act' of that time.
The mark is painted onto the side of cargo vessels to indicate the limit to which they can be legally loaded.
Next time you spend a little bit of time down in the dock area have a look. You will see the mark, similar to that on the left (though without the red writing).
At which marking does the water reach? Can you think of all the reasons that the vessel you are looking at might be at that level?