I've done a simple through hull in a fibreglass dingly and it worked fine. Same should apply to plastic kayak, unless the plastic is double walled or filled with airbubbles like a foam. Just needs to be solid to let the pings go through, but air voids reduce or block the signal. You just have to make some sort of mount  that will hold the transducer in place and prevent any air between the bottom of the transducer and the hull. So some sort of well filled with liquid like water or maybe oil. Plenty of variations in YouTube. Some make a well they fill with water then the transducer sits in that in either a permanent way or a  removeable setup with the well that the tranducer is positioned into then the water poured in. That way they can pull the transducer out to remove the sounder setup and tip the water out etc.