Displacement.
It's a dumb comment assuming it's a slight dig at climate change, but he's not wrong.
Someone else’s response…
As good as no rise at all. Still for the sake of finding out why, lets work it out;
As of 2009 estimate, the world merchant fleet capacity was about 1.192 trillion mt dwt. Thanks to recession in the industry, there has been considerable scrapping so the fleet expansion has not been significant since then.
Still lets calculate the displacement for 1.2 trillion mt
For average sea water density 1025kg/m^3, this works out to be 1.17 trillion cubic meters of water. The surface area covered by ocean is around 3.6E14 squaremeters. (a very rough estimate, from Wikipedia).
Based on above data, the rise in sea level assuming uniform rise over the entire surface comes to just a fraction of a millimeter.
The effect of other ships (warships, private/fishing boats etc) are negligible compared to the merchant fleet size.
Now, some considerations:
With each millimeter rise in sea level, the surface area increases making it difficult to predict.
Sea level rise is not uniform across the earth's surface. Blame ocean currents, Coriolis force and the way oceans are connected. Another reason sea level rises is that oceans are warming up, and this heating is not uniform. Warmer ocean surface is at a higher level than colder ones.
Sea level from warming and melting glaciers comes to barely a millimeter to more than a centimeter in different locations around the world. So effect of ship's displacement is negligible.