They do not "spin" the fuel to get rid of the water. In fact, quite the opposite. The seperater is designed to slow the fuel and water down as such that the water drops to the bottom of the bowl and the fuel is sent upwards so as the water can seperate as efficiently as possible. So no, you will have no issues with too little flow.
And also, it is very easy to see water on the bottom of Diesel
and very easy to see it through the bowl.