How to calculate power from a floating water turbine

I calculate the power you can get from a floating water turbine ,first you need to know the flow rate.This is done by marking off a section of the river for this trial we make off 30 ft on the river bank.
Now throw a floating object about 6 ft in front of this and with a stop watch see how long it takes to trail the 30 ft you might need to repeat this several times to get it right .So if it takes 6 seconds to travail the 30 ft we know the flow is 5 ft a second .
I decided that the size of my fins is 1.5 ft in height and 3 ft in length .

Now to work out the power multiply the height by the length by the flow rate by the fall which =to the depth of the fin . this works whether you have 1 ft or 50 ft of water as its the flow speed makes the power not the dept of water
so 1.5 x 3 x 5 x 1.5 = 33.75 now divide this by 11.8 =2.86 kw this 100% if the machine is working at 75%
multiply 2.86 by .75=  2.14 kw this is the power you can get
I'v looked at some sites and they clam to be getting x power from there turbines when it defy s all calculations ,but then may be their calculations are daily not hour rates
The old saying paper wont refuse ink ?