Hello & Welcome to Just Answer,
Sorry I didn't see your question earlier and to hear of the trouble.
First, let me say "interesting application" - to have this unit in a kayak.
But that brings up a couple of questions
1) What size (capacity) battery you are using to power the system?
2) Do you have an ACTIVE battery monitor - showing the voltage and remaining capacity of the battery - when this device is in use?
3) Is the battery being used for ANYTHING else (such as a small trolling motor, etc.)?
I ask these questions because it "sounds like" the battery voltage is dropping down to a low level - causing the device to shut off.
Please let me know what you have for a power system and specifically if you have a monitor on the battery AND its condition (age, number of charge cycles, etc.).