I can remember a bit of this. You have to use the equation : Velococity = Distance divided by time.
You have to use a device called a hydroprop. You place it in the water and time how long it takes for the propeller to move from one distance to another. Then you meausere the distance and use the equation. This is one of the best ways to do this but you can use many others which may be not as good. such as a ruber duck and time it over a set distance.
Thanks