I have a garmin striker 5. I didn't screw directly to the transom, I made a bracket that I could fasten to the hole already there for the towing eye using a bolt and wing nut. This way I could remove the transducer and wiring from the boat each time I packed it away.
It wasn't affected by the propellor and gave what appeared to be accurate depths even when planing. The bracket held it right by the tube, so as far as possible from the motor.
It could definitely see fish and features on the bottom. Best used with navionics on your phone, so that you find the rough location of the reef using the GPS, and then look out for the exact position on the sonar. However, I would say that just because you can see the fish, it doesn't mean you can catch them. It often shows up big balls of fish for me, but they may be tiny sprats or sandeels, so you can drop your bait into them, but not catch!
A 1lb wrasse swimming back to the bottom:
A thornback Ray swimming down in quite deep water:
Working feathers through a big shoal of nothing! The feathers are the horizontal line, so you can see the depth you are at compared to the fish. In this instance I didn't get anything. The "mark" was maybe small fish.