Oooh.. go on then.. I'll do this one since it's Friday.
1.12bn tonnes of shipping in the world fleet in 2009.
So that's basically 10^12 kg
Mass of world oceans is about 1.4×10^21 kg
So the mass of shipping is 10^-9 x world oceans.
Which is 0.00000001%
Put another way, if you took all the ships out that would be like removing 10^12kg of water from the oceans, or 10^9 m3.
The surface area of the oceans is 3.6X10^8 km2, or 3.6X10^14 m2
So if you spread 10^9 m3 of water over 3.6X10^15 m2 you get a depth of 2.7x10^ -7 m
Which is 270 nanometers.
Or about a tenth the thickness of spider silk.
Given that the tide comes in and out anywhere between 1 and 10m, and even small waves are 0.1m high, you're not going to notice
the sea has a habit of moving about a bit round the edges which would make measuring such a small effect impossible
They measure average sea levels very accurately with satellites. From Wiki:
From 1950 to 2009, measurements show an average annual rise in sea level of 1.7 ± 0.3 mm with satellite data showing a rise of 3.3 ± 0.4 mm from 1993 to 2009,
So they can measure remarkably accurately but not, of course, accurately enough for this.