I have mounted a couple of transducers on Hobies, and agree with CJ's location for more than one reason.
The location behind the sail mast/just before the drive well is a stiff area that in not likely to flex much, if at all.
I chose this location because I use slow cure epoxy 2500 psi, the epoxy is very hard when set and could possible pop off if the mounting location was prone to flex. It has worked flawlessly so far, and has been tested in the pounding ocean off La Jolla during that last Crazy Santa Anna that caught me a few miles off shore. Big nasty mess of wind and waves! It took well over an hour of pounding to get back to shore.
The second reason is when you load a big fish)s in your front hatch they will not interfere with you transducer mount. The sail post acts like a roll bar, and protects the transducer.
Here are some pics.
The Adventure. The transducer is directly behind the sail mast.

Note the Drive Well Strengthening piece is epoxied into place.