Thanks, aerohead! I thought the relevant figure for the Reynolds Number was the viscosity, 13x that of air. If we can really move the water 833x slower than air for a model of the same size, that's great.
My thought was to ignore the flow imaging and just do a/b testing: Put a scale on the tow rope and see how much drag I get on my car model (or any similar bluff body). Then put on miniature turbolator tape in various locations and see if the scale registers a lighter reading for the same water speed.
Then, rig up a waterproof stepper motor to a spoiler and maybe a pump to test the devices in this crazy-looking Chrysler patent:
Motor vehicle with flow-influencing devices to reduce air resistance - US Patent 6378932 Description