Where's the water line?

[cliffpaul2]

I'm getting ready to install a transom mount transducer on a boat that I have never had in the water! The instructions say to use a couple of cable clamps on the wire between the transducer and the hole that passes through the transom. Can someone explain exactly where the water line will be on the transom so I don't drill any holes that are too low? From pictures in the gallery, it seems to be about 2"-3" above the back corners. I have a 305 I/O. Any other tips for this project are welcome too.

Thanks!