as some may have said above, consider the following setup - little bit of effort upfront, but then very easy to setup when you go out:
1. Small sounder - Striker series, for example
2. Install sounder in a "through-hull" configuration. Basically inside your kayak you make a little mould for your transducer, with space all around and above, and silicone it - make sure there are no bubbles :
3. Make a little box for a lifepo4 battery - very very light and will last all day. Make box from a spare tupperware container, modded to pass through cables and sealed with silicone
4. drill hole to allow signal cable through to the top where you would mount/dismount your sounder (i use a spare rod holder slot - modded a PVC tube to hold the sounder).