What you are dealing with is the case of dynamic pressure in fluids. One of your sensors will read the static pressure and the other one will read the total pressure. The difference will be your dynamic pressure. Using the formula: P = q*V˛ you can extract your speed.
P= dynamic pressure
q= density of water
V= speed
So you have your readings for dynamic pressure and you have your water density (I’m not sure if you have to compensate for temperature again since your pressure readings are already compensated) and you can calculate your speed.
I had the chance to do some Google search and I found this:

http://www.engineeringtoolbox.com/dy...re-d_1037.html

HTH.

Regards,

Nick