I would go transom on that boat but if you have to go thru-hull, the transducer needs to be as far back as possible and practical and pretty much on center. What you want to do is keep the transducer in the water when the boat is on plane. Anymore forward that 2 feet or so and the sounder won't work at speed. At the same time, you need to avoid spots where turbulence is active. So stay away from mounting next to or behind water pickups or strainers and off of chines. Finally, if the unit is mounted at an angle not perpendicular to the water, it may not read accurately. The angle of the hull at the mounting point may be an issue.
Hopefully someone on the board has real life experience with the 205. Let us know the model year.
