There is a thru-hull Side Imaging transducer. It takes a 1"+ hole through the hull for attachment and the transducer is on the exterior.
XPTH 9 HDSI 180 T is the model.
It is for slow non-planning hulls.
Some Ranger owners use it under the transom extension and pair it with a shoot-thru 2D transducer XP 9 20.
The trade-in for the HDSI transducer is about $85 including shipping. Then add the cost of shipping the included Compact SI transducer to Humminbird.
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, SOLIX 12 SI, 360, RC-1