I think this could be difficult. You can see certain patterns or fish, but the drone will not float in the water, but lie on the bottom, which will only give a small wave, if any, in the sonar. You must not imagine this as a 3D image of the ground, but only a 2D map as you can see in the product images / videos. It will be difficult to recognise a drone here. You will be more successful if you grab a pair of diving goggles and go splashing around on the off chance. If you want a sonar for fishing anyway, this is a nice gag and also shows some info. But for drone fishing only, I wouldn't recommend buying it.
Hello Ritschre
The Deeper also works in flowing waters.
However, there may be isolated disturbances (impossible depths, etc.) in larger swells.
Otherwise, the thing works tip-top!
Greetings & have fun with it!