truffaldino 
August 9, 2011 16:00 
To accelerate vechile in water you need greater force than in the vacuum even if the steady flow drag is zero, so effect of the water can be seen as a faked mass added to vechile.
The roughest method to suggest to find this additional mass might be to integrate kinetic energy of water (i.e. get total water kinetic energy) multiplied by two and divide it by vechile velocity squared.
However I am not sure this method is completely correct as it assumes quasistacionarity of fluid motion as vechile accelerates. It will not work for sure in 2D for nonzero circulation flow, but perhaps it might be a realistic approximation in 3D.
