You are missing the fuel -- it takes a lot of fuel to drag that parasite across the ocean and it would almost certainly be noticed by the crew of the ship as shipping is a low margin, high volume business and costs are closely watched.
If that parasite is designed slightly aerodynamic (with a total volume of lets say 5-10 tons) and you have a vessel with 100k+ GT, i dont think that the difference in fuel costs will be noticable - the difference due to tide/winds etc. will be much bigger.
Aerodynamics are good. Hydrodynamics are more appropriate.