Here's a little bit about the equations: http://www.engineeringtoolbox.com/pi...bes-d_612.html

You could use the same sensor you used for your depth sensor. And use your depth sensor as the 2nd differential sensor. Connect one to the pitot tube, subtract the readings from the depth sensor, and use the equation. You would need to have the sensors at the same depth, since you can measure a half inch difference. And your depth sensor should probably sense pressure at the side of the hull, I would think.