Depends on if you want transom mount, or in-hull.
For transom mount, likely about 2/3 of the way from centerline towards the tube. Ideally you want to get the transducer away from the motor, and away from roiled water that may come off the tube/hull joint. Do a search on this forum for "transducer" to find ideas for temporary/adjustable mounting to experiment with placement.
For in-hull, you need to have access to the inside surface of the hull (which, I think, many Zodiacs do not.) Towards the rear (say the last quarter of the hull length or so), and somewhat off-center should do. I don't have a whole lot of experience with in-hull mounting.