Humminbird also has many other models that come as portables or can be made portable. Basically any of the PiranhaMax, 300, 500 or 700 series units can be put in a portable case. The Fishin’ Buddy series units (like the 110 that SLABMAGNET mentioned above) is portable as well.

You will want to look at how you will be mounting the unit and its transducer on each boat. We make a kayak transducer mounting kit for some of our transducers: Transducers » Kayak Mounting Hardware
but that would mean you have to buy a second transducer (one for the kayak and one for the jon boat).