Can anybody tell me what the estimated total displacement of all ships and other watercraft is and what the effect on ocean levels would be if all shipping were removed from the Earth's oceans and waterways?

Interesting question. The answer is not obvious.

The best information that i could find that may be relevant is statistics on ship production since 1964 (see http://www.coltoncompany.com/shipbldg/statistics/world.htm ). If we assume that they are all still operative and full of cargo this would add up to 9x10^8 tons (note that GT in the reference means gross tons and not gigaton). This would displace 9x10^8 m^3 of water. If we assume that the area of the oceans is 3.35x10^14 m^2 then these ships raise the level of the oceans by 2.7x10^-6 m or just 2.7 microns.

Not much of an effect.

m4rc... very nicely done with your statistics on sea rise.

Thanks for the answer and the link