On my 2007 620, I mounted my transducer on the steering wheel side of the transom. Never had an issue, I would strongly suggest not using a shoot through hull transducer. Sonar sensitivity will be compromised by, I’ve heard, 20-30 percent. My current boat, 12′ 620, came from the factory with a shoot thru hull transducer, I have it as a back up.
Nate