Just need to make sure you aren't chasing your tail with prop noise as against turbulence. If the picture in shallow is great at speed and only fills with noise as the gain is increased to cope with deeper water, you may not see much difference. If the picture quality reduces at speed no matter what the depth it's worth chasing as that is most likely water flow as against engine/prop noise. If it is gapping (loses the bottom image all together due to air or heavy turbulence) also worth a shot. If it doesn't work perfect first crack with the block, also worth trimming the fairing to create a slight "bow up" attitude for the transducer. You can find the flush mount through hulls sometimes have the slightest lip where the potting compound isn't perfectly leveled as well - this isn't necessarily all the way around the transducer so a slight rotation may provide better water flow (applicable for non tilted element transducers only). The other thing that may also help is fairing the front edge to get rid of the slight step that the water has to travel over - few things to look at / try.