Most of the 2D transducers can be glued to the bottom (inside) section of a plastic kayaks. The signals will pass through the single layer of plastic. The transducer needs to be able lay flat on the plastic with no air gap. Typically mounted down or near the centerline, just forward of the operator, depending on access for SOT. For sit inside, sometimes just behind operator to keep away from feet. If you want SI, that will require an external mount. The new DI units, should be able to work thru the plastic, but not 100% sure if their higher frequencies would be attenuated.
I use to run an external transducer:
http://joefish.homestead.com/Kayak-Drifter.html but have changed to a 3-beam inside the kayak. I can use my color 798ci on it (without SI), but usually use one of my old 400TX's.
Color screens will require larger, heavier, battery packs. Battery packs need to be securely fastened and at a location that will be easy access, and generally not submerged.