You can probably get away with any 12amp battery, as long as you are not running at the highest backlight setting the entire time. I rarely need to use the brightest setting, and usually start the day 3 or 4 notches down. It is easier to see around you without that bright screen in your face (in the dark anyway). You can also extend your day by leaving it off until you need it. No reason to turn it on while the kayak is still on the sand (or even in the harbor or Reserve).
If these suggestions don't work for you, then you will probably want to go with something about 15amps. HDS 7 or 5 around the brightest settings, is going to eat up about 1 amp per hour. I have run one of my LiFePo4 12a batteries dry in 12.5 hours on a late start sunny day, followed by another sunny day (although they are over 5 years old, and very well used
).