Help with water speed please

[Ramius]

Hi all!
I wish to measure the speed of my sub in the water. I am told that a MS4514 sensor with a part number of MS4515-DS-5-A-002-G-P should be a good choice (?) with a range of 0-2 psi. My guess is that with a 10-bit external A/D there should be a decent number representing speed. I know to use "long" as part of the program what I do not know is what formual to use to do the calculation since this is in water and not air! Thanks in advance for any and all assistence!

Best, Ed