I'd agree. 10AH should last you an entire day, if not two.
The first thing you should check is the battery voltage when it quits. I would imagine it would have to be in the low 11's for the Lowrance to quit. Cheap Voltmeters are a dime a dozen anywhere.
If it still has good juice, I'd start checking the connections for corrosion, etc.
If the Lowrance quits randomly, I'd suspect connections. If it consistently quits after 2-3 hours, I'd suspect battery. Do you have a good trickle charger?