Funny you should say that ... that's exactly what I did on the electric racing bike I built, measuring the voltage drop over the longest battery cable and after calibration, displaying it as current. ;-)
The thing with this project though is I want to try to keep it all modular / flexible to it would be better to keep the current measurement 'local' to any meters etc. I'm hoping to use a hall effect module with integrated amplifier chip to make it easier to interface with the Arduino etc.
Also, with mostly 20mm^2 copper leads (2 x 2m) from the battery to the outboard I'm hoping that at my typical 15A draw I'll not get sufficient voltage drop for it to be worth measuring (although I know I will etc). ;-)
Cheers, T i m