I've used an Eagle and currently have a Garmin - both are portable, and I used the Eagle with an over-the-stern transducer on a 16' open boat powered with an outboard - ok when going at very slow speeds but not so good over about 1/3 throttle.
When I transferred it to my old gaff rigger I made a block to house the transducer and stuck it underneath the hull, and it worked perfectly all the time. With the Garmin on my current (grp) boat I have epoxied the transducer inside the hull.
The over-the-stern fitting is very vulnerable - it might be worth mounting on a batten and clamping it to the stern somehow so you can take it off at the end of each trip.